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.ft , (Wojl. OjlX ^JteJL^JUCAj Growt O^-A , 


CLIMATE 

AND WEATHER 

for flight in Naval 
Operational Zones 

☆ 


Prepared by Naval Air Training Command , Pensacola , Florida 

4 


19 44 


Issued by the Aviation Training Division , 
Office of the Chief of Naval Operations , £/. S. iVafy 



\ 


T1RANSFES 

59 

NOV 8 1945 

Corial Recort MvfshJn 
The Library of Congmnr 

C«?y-——-- 


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9 ? 


PREFACE 


This book is intended as a text to serve aviation cadets who have completed previous 
courses in aerology and flight planning. It presupposes an-understanding of weather 
in its relationships to flying. It is designed to broaden the cadet’s horizon and give him 
a background of the climate and weather over the several oceans where a naval aviator 
might fly during the present conflict. The broad concept of causes and effects pre¬ 
sented in this volume will give the cadet a basis upon which to add more detailed infor¬ 
mation gained from experience. It is hoped that the principles set forth will guide 
him in utilizing weather as an ally—using weather to his advantage in conflict and 
avoiding the weather hazards. 

Acknowledgment is made to the numerous organizations and individuals who have 
so generously contributed both time and material used in preparing this text. 


Ill 















































TABLE OF CONTENTS 


Page 

Preface . iii 

INTRODUCTION . ix 

Chapter 1 

CLIMATIC CONTROLS . i 


Latitude . i 

Influence on air temperature. i 

Influence on air circulation. 2 

Land and Water Distribution. 3 

Influence on air temperature. 3 

Influence on air circulation. 4 

Ocean Currents . 7 

Topography . 12 


Chapter 2 

AIR MASSES AND FRONTS OF THE WORLD 


Source Regions and Classification of Air Masses. 13 

Polar continental (cP). 13 

Arctic (A) . 13 

Tropical maritime (mT). 16 

Tropical continental (cT). 16 

Equatorial (E) and polar maritime (mP). 16 

Air Mass Weather . 16 

cP and A . 16 

Winter .,. 16 

Summer . 17 

mT . 17 

North of a subtropical high.... 18 

South of a subtropical high. 18 

East of a subtropical high. 18 

West of a subtropical high. 19 

cT . 19 

E . 19 

mP . 19 

World Fronts and Frontal Weather. 20 

Polar fronts . 20 

Arctic fronts . 22 

Intertropical fronts . 22 


Page 

Chapter 3 

NORTH ATLANTIC . 23 


Weather Hazards and Helps to Aviation—The General 

Pattern . 23 

Temperature and icing. 23 

A relatively mild climate. 23 

East side milder than west side. 23 

Icing levels .*. 28 

Winds . 28 

Pressure systems affect the winds.. 28 

Surface wind directions. 29 

Wind velocity . 35 

Winds aloft . 35 

Cloudiness and precipitation. 39 

Visibility . 39 

Sea fogs worst in summer. 39 

Weather in the Differing Air Masses and Fronts over 

the North Atlantic . 40 

Air mass weather. 40 

Polar continental air . 40 

Tropical maritime air. 41 

Polar maritime air. 41 

Arctic maritime air. 42 

Tropical continental air... 42 

Frontal weather . 42 

Locations of fronts. 42 

Characteristics of warm fronts. 43 

Characteristics of cold fronts. 43 

Characteristics of occluded fronts. 43 

Flying Weather of Special Areas.„. 44 

From North American shores to Iceland....,. 44 

Newfoundland . 45 

Labrador . 46 

Greenland . 46 

Shores of Northwestern Europe. 46 

General flying weather. 46 

Air mass weather. 48 

The Mediterranean . 49 

Air masses of the Mediterranean Sea and 

shores . 50 

General flying weather. 50 

V 









































































Weather Maps and Flights of the North Atlantic. 

Map series illustrating frontal sequence. 

Weather during individual flights.. 

Chapter 4 

TROPICAL ATLANTIC . 

Flying Weather of the West Indian Region. 

Winds ... 

Seasonal wind velocities. 

Land and sea breezes. 

Winds aloft . 

Northers ... 

Weather maps showing norther in the West 

Indies . 

Hurricanes ... 

Nature .,..... 

Origin . 

Season of occurrence. 

Tracks . 

Speed . 

Signs of approach... 

The “Dangerous Semicircle”.. 

Which direction to fly. 

Weather maps showing a hurricane... 

Thunderstorms ..... 

Visibility ..... 

Flying Weather between Tropical South America and 

Tropical Africa . 

Weather in the intertropical front....!. 

Seasonal migration . 

Doldrum weather ... 

The single front... 

Cyclonic waves on the intertropical front. 

Birthplace of hurricanes. 

Tornadoes of the Gulf of Guinea. 

Trade wind weather. 

Haze and the Harmattan. 

Chapter 5 

NORTH PACIFIC ... 

Weather in the Differing Air Masses and Fronts of 

the North Pacific. 

Air mass weather... 

cP—Asiatic ... 

cP—Canadian ..... 

Arctic . 

mP ..... 

mT ... 

Frontal weather. 

Weather Hazards and Helps to Aviation—The General 

Pattern . 

Winds ... 

Surface winds—directions . 

Winds aloft—directions ... 

Wind force . 


Page 


Icing . 104 

Visibility . 107 

Flying Weather of Special Areas. 107 

The Pacific coast of North America—from San 

Diego to Kodiak. 107 

Flying weather of the southern and central 

California coasts . 111 

Flying weather of the Pacific Northwest and 

British Columbia coasts... 111 

Flying weather of the Gulf of Alaska and its 
coasts . 112 

The Aleutian Islands..,. 113 

Cloudiness and ceilings. 113 

Visibility .... 113 

Icing ..... 114 

Winds .. 114 

Seasonal summary . 115 

Contact or on top?. 115 

One more caution.-. 116 

Pearl Harbor-Midway-Wake area. 116 

Flying conditions in mT air of the subtropical 

Pacific ... 116 

Flying conditions in polar air of the subtropi¬ 
cal Pacific . 117 

Flying conditions in cold fronts of the sub¬ 
tropical Pacific . 117 

Flying conditions in warm fronts of the sub¬ 
tropical Pacific . 117 

Weather Maps and Flights of the North Pacific. 118 

Map series illustrating cyclonic sequence. 118 

Weather during individual flights. 128 


Chapter 6 

CENTRAL AND SOUTH PACIFIC. 


The Wind Pattern. 129 

Weather in the Differing Air Masses and Fronts over 

the Central and South Pacific. 132 

Air mass weather... 132 

Tropical maritime cold (mTk). 132 

Equatorial (E) . 134 

Frontal weather . 134 

The intertropical front. 134 

The doldrums . 136 

The subsidiary front. 137 

General flying weather through doldrum areas 

and intertropical fronts. 138 

Polar front . 138 

Fronts of the South Pacific. 138 

Tropical cyclones . 144 

Flying Weather of Special Areas. 145 

Pearl Harbor-Midway-Wake-Guam... 145 

Line, Marshall and Caroline Islands. 146 

The South Sea Islands..... 146 

New Zealand and eastern Australia. 147 


Page 

5 1 

5i 

61 

73 

73 

73 

73 

73 

79 

79 

82 

82 

83 

83 

83 

84 

85 

85 

89 

89 

89 

89 

9 1 

92 

92 

9 2 

94 

94 

94 

95 

95 

95 

95 

97 

97 

97 

97 

100 

100 

100 

100 

100 

104 

104 

104 

104 

104 


VI 



























































































Page 


Page 


Chapter 7 

SOUTHWEST PACIFIC AND INDIAN OCEAN 


Monsoons and the Migration of the Intertropical Front 149 

Monsoons of the northern winter. 149 

Monsoons of the northern summer. 149 

The transition seasons. 152 

The Seasonal Pattern of Weather... 154 

Seasonal pattern of air mass weather. 154 

In winter of the Northern Hemisphere.. 154 

In summer of the Northern Hemisphere. 154 

Transition seasons .,. 155 

Seasonal pattern of frontal weather. 155 

Weather in the intertropical front. 155 

Invasions of polar front activity. 156 

Tropical cyclones .•.. 156 

Effects of Land and Sea Contrasts and Mountain Bar¬ 
riers . 158 

Land and sea breezes.,.. 158 

Katabatic winds ... 158 

Orographic fronts . 158 

Windward coast cloudiness. 159 

Diurnal changes in storminess. 160 

General Summary of Flying Conditions. 161 

Ceilings and visibility. 161 

Winds ..... 161 

Winds aloft . 162 

Flying Weather of Special Areas. 162 

Flying weather from India to Malaya (Bay of 

Bengal) . 162 

Winter monsoon season. 162 

Summer monsoon season. 163 

Transitional period—Spring.. 163 

Transitional period—Autumn.163 

Flying weather from Australia to Malaya. 164 

Westerly monsoon season (Dec. to Mar.). 165 

Southeast monsoon season (May to Sept.). 166 

Inter-monsoon seasons (April and Oct.-Nov.y. 166 

Flying weather of the Philippines and South China 

Sea . 167 

Winter monsoon season (Nov. to Mar.)..... 167 

Summer monsoon season (Apr. to Oct.). 169 


Chapter 8 

JAPAN AND NEIGHBORING SEAS.. 


The Climatic Controls...... 171 

Land and water; and latitude . 171 

Topography ... 171 

Ocean currents ..... 171 

Weather in the Differing Air Masses and Fronts over 

Japan and Neighboring Seas. 173 

Air mass weather... 173 

Frontal weather . 175 

Polar front . 175 

The coastal front.... 175 

Weather Hazards and Helps to Aviation—The General 

Pattern . 176 

Winds . 176 

Surface winds . 176 

Winds aloft . 177 

Typhoons . 177 

Thunderstorms . 178 

Cloudiness and ceilings. 178 

Visibility .;... 179 

Haze . 180 

Precipitation . 180 

Fog . 180 

Temperature and icing. 181 

Weather Maps of Eastern Asia and Japan.. 182 

Winter . 182 

Spring . 182 

Summer . 182 

Autumn . 182 


KNOWLEDGE IS POWER 


GLOSSARY.-... 197 

Beaufort Scale (with Adaptations)..... 201 

HOW TO PRONOUNCE PLACE NAMES. 202 


VII 











































































C^sg- INTRODUCTION -Ss^O 


As a Navy pilot you may fly anywhere—from Kiska to Bali, from Hamburg to Yokohama. Most of 
your operations will be over ocean and coastal areas, and in the course of a naval flying career you 
may operate in any one of several of the naval transport, patrol and combat zones. Many of these 
areas have “flying weather” vastly different from that you have known at home. To use weather as an 
ally, and to avoid or minimize the hazards it holds, you must know something of the “climatic pattern” 
of the earth—the weather possibilities of distant regions. Understanding of world weather will make 
you a more effective Navy pilot. 


In the War Zones, Weather Information Is In¬ 
complete. When operating off carriers, an air crew 
receives no radioed weather reports while in flight— 
for messages radioed from carriers might reveal to 
the enemy the locations of battle units. Even more 
significant, an air crew—whether carrier- or land- 
based—frequently receives only sketchy or general¬ 
ized weather information before taking off on a mis¬ 
sion. 

Outside of continental United States, except in a 
few places, pilots lack the “luxury” of teletype 
sequence reports—no teletype systems serve the 
oceans. Weather maps are drawn on all bases and 
carriers, but these are not the detailed and reliable 
maps that serve continental airways. They cannot be 
detailed. Over large ocean areas the only weather 
information available, in addition to observations at 
the home station, are (i) radioed reports (in code) 
from scattered bases and (2) information brought 
home by pilots on patrol or other missions. Ships at 
sea, the principal peacetime source of ocean weather 
information, do not transmit reports during wartime 
(they feel no particular urge to reveal their locations 
to the enemy); and we receive no weather informa¬ 
tion from territories under enemy control. Bellig¬ 


erent nations—Allied and Axis—guard their weather 
knowledge carefully, for the battle force that knows 
the characteristics of the weather moving over a 
combat zone has a distinct advantage in planning 
attacks. This leaves large gaps in the weather data 
available for weather maps and for use in forecast¬ 
ing. 

Your aerologist, thoroughly trained in the wiles of 
world weather, meets this challenge. He fills the gaps 
with conjectured air mass and frontal weather, bas¬ 
ing the conjectures (or guesses) on an understanding 
of the nature and behavior of the atmosphere and on 
climate. 

Climate Is Weather History. The natural fac¬ 
tors that regulate weather are essentially the same in 
the 1940’s as in the 1490’s. Although one winter may 
be somewhat colder or warmer than the preceding 
winter, and although the 4th of July seldom brings 
exactly the same weather as last year’s 4th, weather 
continues to obey its natural controls and to pulsate 
with the seasons.* 

* Possibly the weather also moves in great cycles (9-year, 11- 
year, or 22-year cycles) not yet fully understood. These cycles, 
however, need not disturb us in this book. 


IX 




Belligerent nations guard their weather knowledge carefully 

All over the earth the same general factors in¬ 
fluence weather. Two places thousands of miles apart 
have similar climatic characteristics if they share the 
same latitude, topographic setting, and relationship 
to a continent (leeward or windward side). Get into 
the habit of comparing California weather with that 
of the Mediterranean shore-lands, and Japanese 
weather with that of our Atlantic seaboard. There 
will always be differences, but the basic pattern is 
the same. 

This book will acquaint you with the climates 
(usual weather or weather histories) of the regions 
in which you will fly. 


In Action, Knowledge of Climate Will Hejp You to Interpret Cor¬ 
rectly the Available Weather Information and Fill in the Gaps 

Your aerologist is an expert, and your weather ser¬ 
vice is good. Have confidence in the aerologist and 
remember that he knows far more than you will 
learn from this book. He’ll do his work well. 

It is your job, however, to understand fully the 
weather information he gives you! To do that 
job well—to get full meanings, in terms of flying 
weather, from the wartime weather maps and fore¬ 
casts of ocean areas—you will find it extremely 


helpful to know something of the general climatic 
pattern of the ocean and coastal areas over which 
you will fly. 

This book includes descriptions of climates, and 
explanations that will help you remember the cli¬ 
matic conditions. It also includes “sample” weather 
maps for naval operation zones. It is difficult to 
select thoroughly “typical” weather maps, but these 
are “typical” in at least one respect: they lack much 
of the data important to flight planning. Because 
weather reports are missing over extensive areas, the 
maps fail to give detailed information on regions 
with fog and low ceilings, thunderstorms, and icing 
conditions. Even the location of fronts and extent 
of precipitation areas is partly guesswork. No wind 
aloft reports or charts accompany the maps. You 
understand 'why these maps cannot contain more de¬ 
tailed information. You will understand, also, that 
in planning flights 'while in action you cannot rely 
alone on the available current weather information. 

You must interpret that information in the light of 
the “usual” weather of the region concerned, then 
supplement it with a knowledge of where weather 
hazards to flight are most likely to occur and a 
knowledge of prevailing winds aloft. 



The natural factors that regulate weather are essentially the 
same in the 1940’s as in the 1490’s 


X 




























The current weather map cannot be looked at 
through a slot in the ruler along your proposed line 
of flight. You never know when a Zero or a Jap car¬ 
rier will coax you off that narrow path. Scan the 
whole area at all operating altitudes. Even when 
your scanty weather map tells you that all is 
CAVU,* knowledge of climate will prepare you for 
weather phenomena most likely to occur. Also, con¬ 
sideration of climate may reveal that one possible 
route is likely to have more favorable flying condi¬ 
tions than another. For example, one side of an island 
may be correctly guessed to be clear, while the other 
side is correctly guessed to be clouded or fogged. It’s 
a challenging chore—this use of climatic knowledge 
to enable you to “correctly guess.” 


In Action, Knowledge of Climate Will Help You to Interpret 
Correctly Your Own Observations 

Weather observations made while in flight, and 
their proper interpretation, are highly important 
to Navy pilots. While flying in regions where 
weather information is incomplete (or even lacking), 
pilots must observe the weather as it passes and in¬ 
terpret what they see. A forecast can go sour even 
in peacetime, and the chances in wartime are much 
more remote of a cumulo-nimbus being where 


* “Ceiling and visibility unlimited.” 



Fill in the gaps. 



you were told it would be. Once a pilot is in the air 
and encounters unexpected weather he is very much 
on his own—the home aerologist can no longer help 
him. The pilot must recognize from the cockpit all 
weather aids and hazards and fly accordingly , adapt¬ 
ing military tactics to weather conditions. Ability to 
observe under standingly the passing weather, seeing 
the hazards it holds and the helps it offers—and flying 
accordingly—saves planes, saves lives and wins wars! 

As a Navy pilot your weather observations will 
be important to many others besides yourself. During 
wartime every facility must be employed to obtain 
weather information. For the success of aviation, all 
airmen must be ready and trained to be weather 
observers. When operating on patrol or bomber 
flights, from a carrier or an isolated base, the reports 
brought back by pilots may serve as the chief source 
of weather information for large areas. You are a 
“weather spy.” All weather information collected 
on flights will be helpful to the ship’s aerologist, 
aiding him in drawing maps and making forecasts. 
Upon the correct and capable observations of 
weather by pilots will largely depend the accuracy 
of maps and forecasts in the naval war zones. The 
reports which you turn in to the aerologist today 
may mean the difference between victory or defeat 
to a fellow pilot tomorrow. 


XI 





/ 





































CLIMATIC CONTROLS 


Air masses are the great actors on the weather stage—they bring all our weather, both air mass and 
frontal. But air masses are puppets . Climatic controls determine their sources, directions of movement, 
and characteristics. They pull the strings that make air masses move. 


The climatic controls, operating together, give 
the earth a general climatic pattern. While studying 
the climates of naval operation zones, make frequent 
mental comparisons with your home region and with 
the other regions studied. Each comparison helps 
memory. For example, Tennesseans bombing Tokyo 
find seasonal temperatures and flying conditions much 
like those at home, for both Tennessee and Tokyo 
are in latitude 35 ° with large land areas to windward. 
Californians flying through Mediterranean skyways 
discover why climatologists call California’s coastal 
climate “Mediterranean”—both of these regions lie 
in subtropical latitudes east of large oceans and both 
have wet winters, dry summers, and moderate tem¬ 
peratures that bring Iowans to Long Beach and 
Parisians to the Riviera. Pilots who have flown the 
fogs and fronts of Newfoundland (45 0 to 50° N. 
Latitude, northeast of the United States) will under¬ 
stand the fogs and fronts of Japan’s Kuril Islands 
(45 0 to 50° N. Latitude, northeast of Japan Proper). 
And pilots who have flown through the “flight 
weather at its best” of the central part of the North 
Pacific, near Hawaii, will discover pleasantly similar 
weather when flying between Bermuda and the 
Azores in the central part of the North Atlantic. 
These suggest only a few examples of earth’s general 
climatic pattern. 

Two principal factors and two secondary factors 
largely determine the climate of every ocean and 
coastal region. They lay the climatic pattern. These 
four climatic controls are: 

Principal: 

1. Latitude 

2. Land and water distribution 

Secondary: 

3. Topography 

4. Ocean currents 


Understanding of these controls will enable you 
to understand and remember the climate and weather 
of naval operation zones. 

LATITUDE 

Latitude influences air temperature , in that dis¬ 
tance north or south of the equator determines (a) 
the angle at which rays of sunlight reach the earth, 
and (b) the number of “sun” hours each day. The 
temperature of an air mass greatly influences the 
flight characteristics of the weather it brings. 

Also, through its influence on air temperature, 
latitude helps determine the sources and directions 
of movement of air masses, for uneven distribution 
of heat sets up the general planetary pattern of air 
circulation. 

Influence on Air Temperature 

The word “climate” is Greek. It literally means 
“inclination”, for early Greeks knew the chief fac¬ 
tor influencing climate to be the inclination of the 
sun’s rays—the angle at which they strike the earth. 

Regions under direct (perpendicular) or nearly 
direct rays of the sun receive more heat (per unit 
of time) than those under oblique rays. From your 
own experience, compare the heat brought by the 
slanting rays of early morning and heat brought by 
the more nearly direct rays of midday—the slanting 
rays of winter and the more nearly direct rays of 
summer. 

Figure 1 shows four positions of the earth on its 
annual revolution around the sun. Notice that the 
earth’s axis (there are others who think they’re the 
axis) is tilted 2^y 2 ° and always held parallel to the 
previous position. Each year, consequently, the per¬ 
pendicular rays of the sun migrate to 2 3 V2 °N. Lati¬ 
tude (summer solstice—June 21st) and 23*4 °S. Lati¬ 
tude (winter solstice—December 22nd). This causes 
our seasons. The warmest weather of the Northern 
Hemisphere, for example, comes a short while 

1 


after June 21st, when the Northern Hemisphere has 
absorbed much heat from a sun that reaches high 
above the horizon each noon. 

Figure 2, more clearly than Figure 1, shows how 
the perpendicular and tangent rays of the sun on 
June 21 st and December 22nd “trace” on earth’s sur¬ 
face the lines we call the Tropic of Cancer (23 1 / 2 °N. 
Latitude), Tropic of Capricorn (23 1 /2°S. Latitude), 
Arctic Circle ( 66 l , 4 0 N. Latitude), and Antarctic 
Circle ( 66/ 2 °S. Latitude). Figure 1 shows the rela¬ 
tionship between the sun’s rays and the earth during 
the spring and fall equinoxes (March 21st and Sep¬ 
tember 22nd). “Equinox” means “equal night.” On 
March 21st and September 22nd, when the sun’s 
perpendicular rays strike the equator and tangent 
rays reach exactly to the poles, every latitude (ex¬ 
cept the poles) has 12 hours of daylight and 12 hours 
of night—“equal night.” 

Length of day, like the angle of the sun’s rays, in¬ 
fluences temperature. Length of day varies with lati¬ 
tude and season. Notice the daylight circle in Figures 
1 and 2. The daylight circle is that great circle that 
divides the earth into a daylight half and a dark half. 
Notice that, as earth rotates on its axis, half of the 
equator always has daylight and half has darkness. 
This is true at all seasons. A place near the equator, 
then, like Singapore or the Galapagos, has about 12 
hours of daylight every day in the year. Because of 
this, and because the sun at noonday is always quite 
high in the sky (giving nearly direct rays), equa¬ 
torial regions have no pronounced seasonal tempera¬ 
ture changes. 

Figure 1 shows that each pole has 6 months of 
daylight every year and 6 months in which the sun 
does not appear above the horizon. On June 21st all 
territory 'within the Arctic Circle (which includes 
Murmansk and northern Alaska) experiences 24 
hours of daylight. On December 22 nd, all this terri¬ 




tory has semi-darkness or twilight, and at such places 
as Reykjavik, Iceland (Latitude 64°N.) the sun 
makes only a brief, uninterested midday appearance. 

During summer in the Northern Hemisphere, all 
places north of the equator have more than 12 hours 
of daylight. On June 21st, the length of day ranges 
from 12 hours at the equator to about 14 hours at 
30°N. Latitude, 15 hours at 40°N. Latitude and 24 
hours at the Arctic Circle ( 66 * 4 °N. Latitude). At 
Kodiak, Alaska, (58°N. Latitude) for example, the 
sun on June 21st rises soon after 0300 and sets at 2100, 
giving Kodiak nearly 18 hours of daylight. During 
winter in the Northern Hemisphere, the situation 
reverses. All latitudes north of the equator receive 
less than 12 hours of daylight. At Kodiak, on De¬ 
cember 22 nd, the sun moves above the horizon for 
little more than 6 hours. It rises about 0900 and sets 
a few minutes before 1500. 

Great seasonal variation in length of day, and the 
seasonal difference in the angle at which the sun’s rays 
reach earth’s surface, cause seasonal temperature dif¬ 
ferences in middle and high latitudes (Figure 3). In 
the far north, long hours of winter darkness produce 
cold temperatures that breed powerful polar air 
masses; long hours of summer daylight thaw lakes 
into landing fields for flying boats, ripen berries and 
mosquitoes, and weaken the polar air masses. 

Influence on Air Circulation 

Uneven heat distribution causes air movement- 
makes wind blow. The latitudinal difference in 
earth’s heating sets air in motion—tends to develop 
a general pattern of motion over all the earth. 
Through its effect on heat distribution, therefore, 
latitude helps determine the sources and directions in 
the movement of air masses. Before mapping the air 
circulation, we must consider the other principal 
climatic control—land and water distribution. 


2 













LAND AND WATER DISTRIBUTION 

Because land and water heat and cool at different 
rates, the location of continents and oceans greatly 
alters the earth’s pattern of air temperature and in¬ 
fluences the sources and directions of movement of 
air masses. 

Influence on Air Temperature 

Water surfaces heat more slowly and cool more 
slowly than do land surfaces. The result is compara¬ 
tively moderate seasonal changes over the oceans, in 
contrast to the extreme seasonal changes over the 
land. In winter, oceanic climates are not so cold as 
continental climates; in summer, the oceanic climates 
are not so hot. The Navy is properly appreciative. 



22 DECEMBER-WINTER SOLSTICE 



Coastal areas take on the temperature character¬ 
istics of the land or water to their windward. In the 
latitudes of the prevailing westerly winds, for ex¬ 
ample, west coasts of continents have oceanic tem¬ 
peratures, east coasts have continental temperatures. 
The temperatures are “imported” by the wind! In 
Figure 4 notice the contrast in the temperature 
regimes of San Francisco (west coast, 38°N. Lati¬ 
tude) and Annapolis (east coast, 39°N. Latitude). 
Of Glasgow, Scotland (west coast, 55°N. Latitude) 
and Cartwright, Labrador (east coast, 54°N. Lati¬ 
tude). Of Prince Rupert, British Columbia (west 
coast, 54 0 N. Latitude) and Petropavlovsk, U.S.S.R. 
(east coast, 53°N. Latitude). (Ocean currents, to be 
considered later, are a secondary factor in controlling 
ocean and coastal temperatures.) 



POSITION OF PERPENDICULAR AND 
TANGENT SUN RAYS DETERMINES 
TROPICS OF CANCER AND CAPRICORN 
AND ARCTIC AND ANTARCTIC CIRCLES 

POSITION OF DAYLIGHT CIRCLE DETER¬ 
MINES LENGTH OF DAY AND NIGHT. 


Fig. 2 


1 


22 SEPTEMBER-AUTUMN EQUINOX 
21 MARCH-SPRING EQUINOX 


3 


























Fig. 3 

Figures 5 and 6, giving average sea-level air tem¬ 
peratures for January and July, show the influence 
of latitude and of land and water distribution on tem¬ 
peratures of oceans and coasts. Studying Figures 5 
and 6, notice that land areas have greater seasonal 
extremes of temperature than do ocean areas of the 
same latitude, and that in latitudes of the prevailing 
westerlies, west coasts of continents have more 
oceanic temperatures than do east coasts of conti¬ 
nents. Much of Great Britain, for example, has an 
average temperature of 40 °F. in January and only 
60°F. in July—an example of marine or oceanic cli¬ 
mate. Figures 5 and 6 also show the migration of the 
heat equator —from generally south of the equator in 
January to north of the equator in July. They show 
the slight seasonal temperature variations of equa¬ 
torial regions and the greater seasonal contrasts of 
areas of middle and high latitude. 


While reading the following paragraphs, refer to 
Figures 7, 8, and 9. 

Equatorial air heats most. Expanded, and light per 
unit volume, this warm air causes an equatorial low 
pressure belt. We commonly call it the doldrums. Air 
blows toward the doldrum belt as toward all lows. 
Within the belt, air ascends. The warm moist air 
ascends convectionally over heated surfaces, produc¬ 
ing thunderstorms; or it is forced to ascend along 
fronts {intertropical fronts) formed by two air 
masses blowing into the equatorial low from the two 
hemispheres. Consequently, the doldrum belt has 
variable light winds with turbulent storms and 
heavy rain showers nearly every day. 

Turn your attention now to earth’s coldest regions. 
Over cold land or ice-covered surfaces the cold con¬ 
tracted air (heavy per unit volume) heavily hugs 
the earth, causing polar highs. Within these polar 
highs, air is in general descent, with variable light 
surface winds and frequent calms. Clear skies pre¬ 
vail, except where low cloud layers develop under 
temperature inversions or where shallow convection 
causes cumulus formation. 



Influence on Air Circulation 

Figure 7 indicates how air would probably cir¬ 
culate, owing to latitudinal differences in heating, if 
earth’s surface were all of uniform substance like 
a spherical Edam cheese. It shows deflection of the 
winds—deflection to the right of the pressure gradient 
in the Northern Hemisphere and to the left in the 
Southern—and it names the idealized or generalized 
“belts” of winds and of semi-permanent low or high 
pressure. These “belts” would of course migrate 
with the seasons, moving a few degrees north of their 
average latitude for the northern summer , a few de¬ 
grees south in the southern summer. Migration would 
be in response to the much greater seasonal migra¬ 
tion of the direct rays of the sun. 

Figures 8 and 9 show the general pattern of air 
circulation much as it actually exists. The major 
factors controlling this circulation are (1) latitude 
and (2) land and water distribution. Notice how the 
uneven heating and cooling of continents and oceans 
disrupts the simple latitudinal pattern of Figure 7. 

4 




Fig. 4. Contrasting temperatures on eastern and western coasts 










































































5 






















































Notice that the polar high of the Southern Hemi¬ 
sphere lies over the ice-covered continent of Antarc¬ 
tica (Figures 8 and 9). In the Northern Hemisphere, 
high pressure frequently forms over the Arctic pack 
ice but the northern part of the great ice-crowned 
bulk of Greenland boasts the most persistent polar 
high of the hemisphere. In winter of the Northern 
Hemisphere the cooling of the great land mass causes 
a large Asiatic high, and a significant high forms for 
a similar reason over northern North America. In 
summer these two continental polar highs dissipate 
owing to the warming of the land. 

Polar highs are the source of polar (and Arctic) 
air. If the equatorial low and the polar highs com¬ 
pletely dominated air circulation, two great vertical 
circuits would provide our out-of-door air condition¬ 
ing. In each hemisphere cold polar air would blow 
equatorward, warming as it goes; in equatorial re¬ 
gions it would ascend; it would then move poleward 
and descend over polar areas. But the pattern is not 
that simple. 

Subtropical highs add the master touch to the pat¬ 
tern of general circulation. At about 30 °N. Latitude 
and 30°S. Latitude, air descends in areas or “cells” 
of semi-permanent high pressure. These highs com¬ 


monly have flying weather at near its best, with skies 
seldom overcast and with light, though variable, 
winds. From these subtropical highs, as from all 
highs, winds spiral outward— clockwise in the North¬ 
ern Hemisphere, counter-clockwise in the Southern. 
These subtropical highs are prolific sources of tropi¬ 
cal air. 

Figures 8 and 9 reveal five year-around subtropical 
highs over ocean areas. These maritime subtropical 
highs, sources of maritime tropical air, occupy the 
South Indian Ocean and eastern portions of the At¬ 
lantic and Pacific in latitudes about 30°N. and 30°S. 
Subtropical highs of the North Atlantic and North 
Pacific are largest and best developed—spread good 
flying weather over broadest areas—in their summer 
season, for then the oceans are cool in comparison 
with neighboring continents. But, though best de¬ 
veloped in summer, the subtropical highs remain 
through all seasons. 

Figures 8 and 9 show two seasonal subtropical 
highs of continental areas—the Sahara and Australia. 
These form in the winter of their respective hemi¬ 
spheres, for then the continents are cool in com¬ 
parison with neighboring oceans. 


Winds easterly at surface 
Westerly aloft 


Polar hiqh 


Winds westerly 
at all elevations 

Winds easterly 
to 5,000 ft. - 
then westerly A 

Winds easterly / 
to 25,000 ft. - I ft 
then westerly '\'r1r£} 

Winds variable 


Polar Front 

^Gloud top 12,000 ft.± 
'^.Ceilinq below 1,000 ft .'- 

..Stratus 

tops 4,000 ft.± 
ceilinqs 2,OOOft.i 


8,000 ft. ± 
Cumulus 



ceilinq 2,000ft.± 

Intertropical 
Front 

03$ Cumulo-nimbus 
tops 30,000 fti 
ceilinq below 1,000ft. 


Polar Hiqh 

Fig. 7. Idealized pattern ot atmospheric circulation 


6 









Tropical surface air blows equatorward from the 
subtropical highs as trade 'winds. Owing to deflec¬ 
tion, these are NE. trades in the Northern Hemi¬ 
sphere, SE. trades in the Southern. The trade winds 
maintain more constancy of direction than any other 
winds of earth, having earned their reputation as de¬ 
pendable winds at the time when trading vessels bore 
sails. They are interrupted, however, by three types 
of disturbances: (i) occasional frontal activity, 
especially in winter when polar air may pour far 
equatorward; (2) tropical cyclones; and (3) mon¬ 
soons, which in the tropical oceans near Asia com¬ 
pletely disrupt the trade winds, reversing the wind 
direction during the summer season. 

As the (typical) trade wind air moves equator- 
ward, it grows warmer; and thus the air absorbs 
rather than loses moisture. Shallow convection pro¬ 
duces scattered fair-weather cumuli but little rain. 
Only where trades blow over coastal or island moun¬ 
tains does much precipitation occur, and there great 
cloud banks may hide the mountains while rain 
pours. In general, the trade winds, when free from 
occasional invading storms, rank with subtropical 
highs as producers of good flying weather. 

Throughout the island area between Asia and Aus¬ 
tralia, and over the North Indian Ocean south of 
India and Burma, the great Asiatic monsoon wind 
system rules. “Monsoon” means seasonal. Winds 
blow away from Asia during the northern winter, 
when the huge and persistent continental high re¬ 
sults from the cooling of the land surfaces; but blow 
toward Asia during the northern summer, when low 
pressure develops over the summer-hot land. Figures 
8 and 9 show that the outward-blowing winter mon¬ 
soons reach a doldrum belt that extends from the 
equatorial belt of the Indian Ocean to the seas and 
shores of northern Australia. They show that the 
inward-blowing summer monsoons originate in the 
maritime subtropical high of the South Indian Ocean 
(about 30°S. Latitude) and in the Australian conti¬ 
nental subtropical high of the Australian winter sea¬ 
son. Note that in summer there are no NE. trade 
winds in the North Indian and Southwest Pacific 
Oceans; instead, the wind direction has been exactly 
reversed by the monsoon influence. 

Prevailing westerly winds dominate the latitudes 
just poleward from the subtropical highs. According 
to the generalized circulation as shown in Figure 7, 
the air of the prevailing westerlies originates in the 
subtropical highs. Figures 8 and 9 reveal, however, 
that much of the air that blows as prevailing westerly 
wind comes from polar sources (from highs over 
Asia and North America) especially in the winter 


season. Farther poleward, blowing from the great 
polar highs, Arctic air masses advance as polar 
easterly winds.* 

Where polar easterly winds meet prevailing 
westerlies, Arctic fronts or polar fronts develop. 
Polar fronts also form within the belt of prevailing 
westerlies where air from the polar highs meet air 
from the subtropical highs. Low pressure troughs 
form along the polar and Arctic fronts and cyclonic 
lows move from west to east along the fronts. Here 
atmospheric turmoil prevails and weather forecasters 
must work for their pay. 

Figures 8 and 9 show two great winter low pres¬ 
sure areas over northern oceans—the Icelandic low 
over the northern Atlantic and the Aleutian low 
over the northern Pacific. Winds spiral inward, 
counterclockwise, toward these lows. Remember 
that winter low pressure in these areas, like every¬ 
thing on Figures 8 and 9, is the average condition. 
Low pressure is prevalent in these two regions be¬ 
cause (1) the oceans of this latitude are much warmer 
in winter than the continents and (2) although the 
polar front of winter often extends far equatorward 
over the continents, most of its low-pressure storm 
centers, and most of those of the Arctic fronts, cross 
the oceans on northern routes and bring frequent 
storms to the Icelandic and Aleutian regions. 

Figure 7 indicates more air moving into the polar 
highs than out of them. That can't be right. Balance 
is maintained by occasional or periodic polar air 
“outbreaks” through the polar front—outbreaks 
in which polar air pours equatorward between the 
subtropical high pressure cells or areas and, joining 
the trade winds, flows on to the doldrum belt. 

OCEAN CURRENTS 

Moving water obeys the same law of deflection as 
moving air—it deflects toward its own right in the 
Northern Hemisphere, toward its own left in the 
Southern Hemisphere. Earth’s rotation provides basic 
cause for this pattern of deflection; but the prevail¬ 
ing winds, exerting pressure along the ocean surface, 
stimulate the ocean circulation and determine some 
of its characteristics. As a consequence of the deflec¬ 
tion, great clockwise ocean eddies occupy the North 
Atlantic and the North Pacific, while waters of the 
Southern Hemisphere swirl in counterclockwise 
fashion (Figure 10). Knowing this rule, you can de¬ 
termine the general direction of ocean current off 
most coasts of middle latitudes. Figure 10 shows the 
general pattern of ocean currents and names the 
principal currents. 

* Though Figures 8 and 9 do not show it, polar easterly winds 
blow outward from the Antarctica high also. 


7 



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Fig. 9 













10 


























In low latitudes, North Equatorial and South 
Equatorial currents flow westward.* Between these 
westward-flowing currents, in the belt of the dol¬ 
drums, Equatorial Counter Currents flow eastward. 
The westward-flowing equatorial currents are stimu¬ 
lated by the trade winds exerting a westward stress 
on the sea surface. This westward stress produces a 
westward ascent of sea level along the equator. 
(Don’t think you’re going to change altitude—it’s 
a matter of inches.) The eastward-flowing Equatorial 
Counter Currents apparently result simply as down- 
slope flows in the doldrum belt where the trade winds 
are absent. 

The North Equatorial Current of the Pacific, 
reaching the western Pacific, turns northward then 
northeastward to bathe Japanese shores as the Japan 
Current. In middle latitudes, this water crosses the 
Pacific as the North Pacific Current. Between the 
Hawaiian Islands and North American shores, part 
of it turns southward and eventually rejoins the 
North Equatorial Current. From the Bering Sea and 
the Sea of Okhotsk, cold water moves southward 
along Siberian coasts; it moves into the Sea of Japan 
and along the Kuril Islands. North of the North 
Pacific Current, some of this colder water crosses 
the ocean. Arriving near North American shores, 
part of it turns southward as the California Current 
and joins the North Equatorial Current, while part 
turns northward as the Alaska Current. 

A similar pattern of currents occupies each of 
the other great oceans. In the North Atlantic, how¬ 
ever, the broad opening into the Arctic Ocean and 
the comparative narrowness of the Atlantic give 
the Gulf Stream system a northward sweep along 
the Norwegian coast that has no counterpart along 
the Alaskan coast of the Pacific. (This gives Nor¬ 
way a much milder climate than Alaska has in 
similar latitudes.) 

Ocean currents affect drifting life-boats, and their 
temperatures determine the possibility of sharks. 
Their principal significance to fliers, however, lies 
in the influence on climate and weather. The cli¬ 
mate of an ocean cannot be divorced from its 
currents. 

Currents in Figure io are designated as warm or 
cold. Water in a so-called warm current is rela¬ 
tively warm—relative to the temperature it would 
have if staying permanently on the spot. Warm 
currents are moving from equatorial regions or 
have done so recently. Actually, by the time the 
water from the warm Gulf Stream has crossed the 


* Except north of the equator in the Indian Ocean, where the 
monsoon winds, greatly influencing direction of ocean currents, 
produce seasonal changes. 


Atlantic to Scotland’s coast (Figure io) it has be¬ 
come too cool to please sharks. (And too cool to 
please bathers.) The so-called cold current off the 
Moroccan (North African) coast has warmer tem¬ 
peratures than Scotland’s coastal water, but its 
water is colder than it would be if it had been 
standing off the Moroccan coast throughout the 
year. Cold currents are moving from higher (colder) 
latitudes or have done so recently. 

In middle latitudes, ocean currents carry warm 
water away from the equator along the eastern 
coasts of continents, and carry cold water toward 
the equator along the western coasts (Figure io). 
Winds of the middle latitudes, you will recall, pre¬ 
vail from the west. During winter, the prevailing 
westerly winds bring cold continental air (out¬ 
breaks of polar continental air) over the warm 
waters of the Gulf Stream and over the warm 
waters of the Japan Current. Where this cold conti¬ 
nental air moves over the warm water off eastern 
coasts it creates the most active frontal zones of the 
winter season. 

Here’s how it works. When cold continental air 
moves over the warm water, its surface layers warm 
and absorb moisture from the ocean. Warming and 
absorbing moisture over the warm water, the air 
becomes unstable. Fresh outbreaks of cold air from 
the continent under-run the warmed air, causing 
frontal activity. Maritime, as well as continental, air 
rushes toward the frontal trough. Much condensa¬ 
tion occurs. Condensation releases heat energy into 
the atmosphere and the abundance of released heat 
energy makes the frontal activity exceptionally 
vigorous. 

For similar reason, winter fronts also form where 
cold air masses from Arctic ice-caps blow onto the 
relatively warm water near Iceland, and where cold 
air masses from Alaska-Canada or Siberia blow onto 
the relatively warm water near the Aleutians. 

In middle latitudes, off west coasts of continents, 
cool ocean currents provide insufficient contrast with 
cold winter land to stimulate strong frontal activity. 
In summer, when continents warm, land and water 
of all middle latitudes have insufficient tempera¬ 
ture contrasts to stimulate strong frontal activity. 

Ocean currents also affect the weather within 
a moving air mass. An air mass, moving over an 
ocean surface warmer than the air, tends to form 
cumulus clouds and produce thunderstorms. An air 
mass, moving over an ocean surface colder than the 
air, tends to form stratus clouds or spread extensive 
fogs over the ocean. The latter accounts for much of 
the Newfoundland and Aleutian fog and the coastal 
fog of California. 


11 



TOPOGRAPHY 

Physical features of the land influence the climates 
of coastal areas and islands. 

When air is forced to ascend over mountains, 
clouds may hide the mountains while precipitation 
pours. This happens where trade winds blow over 
such mountainous islands as the Hawaiians, and it 
happens where westerly winds climb west coast 
ranges. Windward slopes often have clouds and pre¬ 
cipitation, while clear flying weather prevails to the 
leeward. Mountains also retard the movement of 
fronts, and may hold storm conditions for several 
days on windward sides of long coastal ranges, 
accentuating the bad flying weather of windward 
regions. 

All fliers with experience in mountainous regions 
know the turbulence caused by air moving over 
mountains. Mountainwise pilots never fly close to a 
mountainside or summit because they know that air 
bumps can lift or drop a plane with equal ease. They 
know the dangerous downdrafts often encountered 


when approaching a range from the leeward. And 
they know the strong winds that frequent? mountain 
valleys. Such fliers will not be surprised by the gusty 
gales that sometimes sweep over, around and between 
the mountainous Aleutian Islands. 

Naval pilots encounter another type of disturbance 
near mountains. Where a high plateau stands near the 
sea, the air over the plateau may become very cold 
by radiation cooling. This cold air then flows down 
the slope as a wind that may attain great force and 
hinder flying. Such winds are called fall-winds, kata¬ 
batic winds or gravity winds. Fliers experience fall- 
winds near all mountainous coasts, and these winds 
become most pronounced where and when high 
pressure forms over a snow or ice-covered upland 
near a relatively warm sea. If the slope is steep, as on 
the coasts of Greenland and Antarctica, the flow be¬ 
comes a cataract of cold air. “Falling” off glaciers, 
the Knik wind of Alaska’s Matanuska Valley and 
the Taku wind near Juneau rip roofs and uproot 
trees. 


AIR MASSES AND FRONTS OF THE WORLD 


You will fly mostly in air mass weather. You will fly through the slowly subsiding, mildly stirring air 
of air mass source regions, and through winds and weather caused by air masses in motion toward fronts. 
Air masses spread helps and hazards over broad areas; also, air mass characteristics determine the nature 
of frontal weather. Pilots of world airways study world air masses for the same reason that pilots of 
American airways study American air masses—to learn the flying conditions within each air mass 
and within the stormy frontal zones where they meet. 


Fronts contain the greatest concentrations of 
weather hazards to flight. Over the oceans you will 
encounter fronts somewhat different from those you 
have known in continental areas. In the tropics you 
will encounter fronts different from those of cooler 
latitudes. 

SOURCE REGIONS AND CLASSIFICATION 
OF AIR MASSES 

You will recall that the term “air mass”, as used 
by aerologists, means a mass of air with horizontal 
homogeneity; that is, physical properties more or 
less uniform at each level. To acquire uniformity or 
homogeneity an air mass must remain over a uniform 
surface for a sufficiently long period of time to 
acquire characteristics of the surface. Two condi¬ 
tions, then, are required to create an air mass: 

1. The general circulation system must frequently 
permit air to linger or remain relatively quiet for a 
few days, and 

2. The underlying surface must be uniform. 

These two conditions occur in the great semi-perma- 
nent high pressure areas—in the polar continental 
highs over snow and ice-covered surfaces, the sub¬ 
tropical maritime highs over oceans of uniform sur¬ 
face temperatures, and the subtropical continental 
highs over Saharan and Australian desert landscape. 
Notice the locations of these semi-permanent highs 
in Figures 8 and 9. Concentrating on the Northern 
Hemisphere, compare them with the principal air 
mass source regions as shown on Figures 11 and 12.* 

* Owing to scarcity of information, and because they do not 
directly influence the principal naval war zones, polar air masses 
of much of the Southern Hemisphere are omitted from this dis¬ 
cussion and from Figures 11 and 12. 


They are identical. While reading the following 
paragraphs, refer frequently to Figures 11 and 12. 

Polar Continental (cP) 

cP air of winter originates in the persistent high 
pressure area that develops over cold Asia during the 
northern winter, and in the winter high that fre¬ 
quently forms over the frozen tundras and timber- 
lands of northern North America. From the Asiatic 
high, winter cP air pours seaward as monsoon winds 
over the North Indian and Southwest Pacific oceans, 
and it surges onto the North Pacific to form the 
polar front that brings some of the adverse flying 
weather to that area. The Asiatic cP air sometimes 
moves north to the Arctic Siberian coast; and, from 
an occasional European extension of the great Asiatic 
high, cP air may advance to the coasts of Europe. 

From winter-cold northern North America, cP air 
masses move southward over the Great Plains and 
seaward to the North Atlantic. 

In the summer of the Northern Hemisphere, as the 
continental highs disappear over the warming land, 
the sources of cP air move farther north and become 
quite feeble. 

Arctic - (A) 

Arctic air of the winter season, though sometimes 
indistinguishable from polar air, is commonly colder. 
It originates over the Arctic icecap and in the great 
polar high that hugs the Greenland icecap. Figures 
11 and 12 show that no distinct front separates Arctic 
air from the cP air of North America. Though Arctic 
air brings the severest cold waves of the Middle 
Western winters, Arctic air masses and cP air masses 
are often quite similar in the American area. Frontal 
activity between Arctic air and polar continental air 

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does occur, however, along the Arctic coast of 
Eurasia. And Arctic air clashes with polar maritime * 
air along fronts of both the North Atlantic and 
North Pacific oceans. 

Tropical Maritime (mT) 

mT air masses have their sources in the maritime 
subtropical high pressure cells or areas located over 
the oceans at about 30° Latitude. (Compare Figures 
8 and 9 with Figures 11 and 12.) From these sources 
the mT air masses move outward in most directions. 
Along the intertropical front they clash with tropical 
air masses of the opposite hemisphere. Along polar 
fronts they meet cooler (polar or Arctic) air masses. 

Tropical Continental (cT) 

cT air originates in subtropical high pressure areas 
that extend over continents. The Saharan high of 
the northern winter sends cT air equatorward to the 
intertropical front and northward to frontal activity 
over the Mediterranean or, occasionally, to frontal 
disturbances over Northwestern Europe. In summer, 
this source of cT air extends farther north. In the 
southern winter a continental subtropical high over 
Australia pours cT air northward toward the sum¬ 
mer low that develops over Asia, and sends cT air 
southward to meet polar air from the South Pacific. 

Equatorial (E) and Polar Maritime (mP) 

Though the great semi-permanent (and/or sea¬ 
sonal) highs are the principal original sources of air 
masses, air may remain over certain other regions 
long enough to acquire characteristics of the under¬ 
lying surfaces. This occurs most frequently in two 
distinct types of regions: 

1. Over equatorial oceans stagnated or slowly- 
moving air masses become modified to equatorial 
(E) air. 

2. Over central and eastern portions of the North 
Atlantic south of Iceland, and over central and east¬ 
ern portions of the North Pacific south of Alaska, 
stagnated or slowly-moving air masses become modi¬ 
fied to polar maritime (mP) air. This polar maritime 
air may originally have been either polar continental, 
Arctic, or tropical maritime. 

AIR MASS WEATHER 

In an air mass source region the air is warm or cold, 
dry or moist, depending upon the nature of the 

# This polar maritime air of the central and eastern portions 
of the North Atlantic and the North Pacific Oceans is generally 
modified polar continental air, sometimes modified Arctic air, 
rarely modified tropical maritime. 


source region. An air mass starts its migration im¬ 
pressed by the characteristics of its source. After 
leaving its source, an air mass is modified by the sur¬ 
face over which it passes. The length of time in 
passage influences the amount of modification. When 
considering weather within each type of air mass, 
then, we must consider (1) weather within the air 
mass at the source region and (2) weather within the 
air mass while in transit over differing surfaces. This 
chapter will present a general world picture of types 
of air mass weather (and, later, types of frontal 
weather). 

cP and A 

Winter. In their source regions, the polar con¬ 
tinental and Arctic air masses of 'winter are cold 
and dry. The air is cold because of contact with the 
cold surfaces, and dry due to absence of water 
surface. 

When the surface air is intensely cold you often 
find warmer air aloft. The reasons: subsiding or 
slow settling of air in the high pressure source regions 
causes adiabatic warming of the air aloft. This, plus 


NEAR SOURCE OF COLD CR 
/TOFTEN /S WARMER UPSTAIRS 



16 






cP air of winter becoming modif ied 
(over warm ocean) to unstable m P 



cP air of winter becoming modified (over cold ocean) 
to stable mP 



Fig. 13 


Fig. 14 


the extreme cooling of the lower air by contact with 
cold surfaces, causes pronounced temperature inver¬ 
sions. These inversions not only cause warmer air 
aloft—they also make the air stable and the air lanes 
smooth. 

Though occasional stratiform cloud layers develop 
and, over the Arctic, steam fog “like smoke from 
many forest fires” billows upward from open water 
exposed by recent breaks in the ice, clear skies 
dominate the winter weather of these source regions. 

As polar continental (or Arctic) air of winter 
moves out of its source regions, its changes depend 
upon the nature of the surfaces over which it travels. 
Over cold land areas this air modifies little, remain¬ 
ing cold and clear and stable. When it advances over 
a snow-free and warmed United States, however 
(or over a warm-surfaced southern China), the lower 
air warms. Consequently, thermal turbulence de¬ 
velops in the lower layers. You’ll find scattered 
cumulus clouds in warmer sections, and strato- 
cumulus where winds move over rough country. 
If the air picks up sufficient moisture, night fogs 
(radiation fogs) may become a nuisance in stagnated 
winter cP air.* 

Moving from land to sea, winter cP air rapidly 
becomes, modified to mP air. Over a warm sea, such 
as the Gulf of Mexico or the China Seas, the cP air 
changes into a moist, unstable, cloud- and rain¬ 
bearing mass with much cumulus development and 
showers (Figure 13). Over a cold sea, such as the 
North Atlantic off Labrador and the North Pacific 
off Kamchatka, the cP air modifies to a stable mP 
with much low strato-cumulus cloud formation, and 
poor visibility caused by frequent rain, drizzle and 
light snow flurries (Figure 14). 

Summer. The coldest air masses in summer are 
those that form over Arctic fields of snow and ice— 
the Arctic icecap and the Greenland glacier. Though 
Arctic summers have predominantly clear skies, both 


* Even Arctic air is generally labeled cP when it travels far 
from its source and undergoes the resultant modification. 


cloudiness and fogs are worse than in the winter 
season. Low stratiform clouds frequently form in 
Arctic summer air owing to a shallow inversion layer, 
for Arctic summer air is extremely chilled only in 
its surface layers. A fog known as “Arctic sea smoke” 
sometimes forms when very cold Arctic air moves 
over warm water, but the more common advection 
sea fogs that you’ll often find along Arctic summer 
coast lines develop when warmer—not Arctic—air 
moves onto cold water. 

The cold surface layer of Arctic summer air warms 
gradually as the air moves southward over open 
sea or onto continents. Some of the cP air of summer 
was originally Arctic air but has become changed to 
cP by an extended sojourn over a thawed-out north- 
land. Some of that summer cP air of Arctic origin 
was first modified to mP air over a cool ocean before 
moving onto the land. For that reason, summer cP 
air masses may exhibit all degrees of transition from 
cold and stable to warm and unstable. Siberian cP 
air of summer is usually cloudless, while the more 
moist European cP air usually has more cumulus 
clouds than that you’ve known over North America. 

Where summer cP air moves over warmer land 
surfaces, daytime turbulence and cumulus clouds 
increase; where it moves over warm ocean surfaces, 
frequent showers develop. Overland, nighttime tem¬ 
perature inversions cause fog whenever cP air carries 
sufficient moisture. 

mT 

In tropical maritime air at its source, within the 
subtropical highs, you’ll find flying weather at near 
its best. Scattered cumulus and thin patches of strato- 
cumulus clouds may develop, but skies are almost 
never overcast and the scant precipitation falls in 
scattered showers. Variable, mild winds prevail. 

The excellent flying weather of these mT source 
regions commonly extends through the moving air 
masses some distance from the sources. Cloudiness 
of the mT air increases, however, with increase in 
distance from the source. If you fly from Hawaii or 

17 

























mT air moving northward over cold ocean 



Subtropical High COLD OCEAN 


Fig. 15 

from the Azores northward through northward-mov¬ 
ing mT air, you’ll find stratiform clouds, increasing 
to the north. If you fly from Hawaii or the Azores 
southward through southward moving mT air (or 
the NE. trades), you’ll find an increasing amount of 
cumulus clouds.* 

North of a Subtropical High. Any mT air that 
moves northward becomes chilled over the cool ocean 
surface. A stratus overcast may form and drizzle fall. 
Far to the north, low ceilings (usually below 1000 
feet) may drop to the surface, producing fog (Figure 
15). The mT air surges farthest north in summer, for 
then subtropical highs are best developed and polar 
fronts lie farthest north. This mT air brings most 
of the summer fogginess of northern seas and coasts. 
It brings greatest fogginess where it blows from over 
the warm Gulf Stream to over the cold Labrador 
Current (near Newfoundland), and where it blows 

• We are here considering only Northern Hemisphere situa¬ 
tions. A comparable pattern of weather exists in the Southern 
Hemisphere. 


from over the warm Japan Current to over the cold 
Kamchatka Current (near Kamchatka). 

South of a Subtropical High. Where the mT 
air moves southward or southeastward (as trade 
winds), its lower layers are warmed by the tropical 
ocean surface. This produces scattered cumulus 
clouds (Figures 17 and 18). Near the equator, after 
absorbing much moisture and being much heated, 
this air may boil up into cumulo-nimbus. 

East of a Subtropical High. Along the Cali¬ 
fornia coast and along the Atlantic coast of North 
Africa, the mT air blows from the west and the 
northwest (Figures 11 and 12). This air tends to 
remain stable* You’ll find the airways usually smooth 
and the skies clear to partly cloudy throughout the 
year. The clouds are generally patches of strato- 
cumulus, and rain is rare. Chief flight hazard in this 
air is coastal fog, which often hides the California 
coastline when mT air chills as it blows from sea 
to land over cold (upwelling) coastal waters. In the 
winter season, when this air chills over cool Cali¬ 
fornia or European land, stratus or strato-cumulus 
clouds may overcast the sky, develop low ceilings, 
and yield a drizzle that reduces visibility. 

* Reasons for remaining stable: 

(1) It is coming from the northern, cooler portion of the 
source region. 

(2) Its surface layers are remaining cool because of moving 
over cold ocean currents (Figure 10). 

(3) Its upper air is warming adiabatically because it con¬ 
tinues to subside. 



Fig. 16. Task force units push past Kiska (U.S. Navy photo) 


18 















mT air moving as TRADE WIND over tropical ocean 

- 8000 ft- 

Hj ^ 

-2000ft.-'^-^-^ 


WARM OCEAN Subtropical high 


Fig. 17 

West of a Subtropical High. The mT air blows 
from the east and the southeast (Figures 11 and 12). 
Blowing over warm water all of the way, the air 
neither chills nor heats. Over the seas near the 
Philippines (and near Florida and the West Indies) 
this trade wind brings good flying weather—clear 
or with scattered cumulus clouds. Moving over 
heated land (for example, southeastern United States 
in summer), this warm moist air becomes unstable 
and turbulent—breeds thunderstorms. Moving over 
cold land (for example, southeastern United States 
in winter), it becomes stable and produces stratus 
clouds or deep fogs. Sweeping northward over cool 
ocean surfaces (as the Sea of Japan and the oceans 
off Kamchatka and Newfoundland), it develops the 
persistent low stratus and fogs for which those areas 
hold world records (Figure 15). 

cT 

Tropical continental (cT) air affects the climate 
of the Mediterranean region and of the Australian 
area. At its source, cT air is warm, dry and stable. It 
affords excellent flying weather with clear skies, and, 
except for haze, good visibility. In winter, cT air 
moving northward from the Sahara is hot and dry 
on the north coast of Africa. It picks up moisture 
over the Mediterranean and arrives at the north 
shores of the Mediterranean as the humid and oppres¬ 
sive sirocco. In summer, cT air from North Africa 
or the Near East can also become oppressive by 
picking up moisture from the Mediterranean, and 
can carry heat waves as far north as England. 

“Down under,” cT air from the interior of 
Australia arrives on the Australia coasts as hot, dry, 
dust-laden winds. In southern Australia, the winds 
from the interior are known as brick fielders. To the 
south and east of Australia they meet fronts. cT air 
moving north out of Australia quickly picks up 
moisture over the tropical seas and modifies to 
Equatorial air. 

E 

This wins the Navy “E” for combat flying 
weather, if you can handle it—if you’ve enough 


lying know-how to dodge thunderstorms. The 
name, equatorial, is applied to any air that lingers 
in an equatorial area long enough to become hot 
and humid. You will encounter it in any equatorial 
region, but in greatest quantities in the Southwest 
Pacific. (The following paragraph summarizes perti¬ 
nent facts about flying weather in this air mass, but 
it’s merely a preview of what you’ll learn about 
equatorial air in Chapter 7.) 

When equatorial air blows over mountains it 
causes much cloudiness and heavy rains on windward 
slopes. When the air heats over hot land or warm 
water surfaces it develops towering cumulus and 
cumulo-nimbus clouds that yield heavy rainfall- 
thunderstorm type. (Pilots who have taken off from 
carriers into these equatorial showers speak of “fly¬ 
ing into a wall of water.”) Though clouds on wind¬ 
ward slopes may be stratus, tall and turbulent cumuli 



Fig. 18. Cumulus clouds between Wake and Guam, 
from 8000 feet 


are the dominant clouds in this air mass. These tall 
cumuli are particularly prevalent over the ocean by 
night and over land in the afternoon. (Except when 
tactics demand it, do not fly in clouds—except for 
short periods necessary to climb, or let down, through 
a stratus.) Over the ocean night flyers have difficulty 
dodging thunderstorms. Best ocean flying in equa¬ 
torial air will usually be found during the day, and 
(to avoid turbulence) above 10,000 feet. 

If the equatorial air penetrates to beyond 35 0 N. 
Latitude, as it may in summer along the Pacific 
Coast of Asia, it becomes rapidly cooled by contact 
with the sea surface, producing dense and persistent 
fogs. 

mP 

When a polar continental or Arctic air mass moves 
out over a cool ocean and remains for a considerable 



19 







time in that environment, it becomes modified to 
polar maritime (mP) air. 

When on duty off the west coast of North 
America or over the seas west of Europe, you’ll be 
in mP air at least 75 per cent of the time. Because 
of the general west to east movement of air masses 
in middle latitudes, mP air is most characteristic of 
these ocean areas off the west coasts. Middle latitude 
oceans off east coasts normally have cP air—cP air 
that is rapidly modifying to mP as it advances across 
the oceans. Sometimes the mP air off an east coast 
circles back to the land, as when a “nor’easter” brings 
mP air to New England. Normally, however, it 
advances across the ocean. 

Weather within an mP air mass, as in any air mass, 
depends upon the temperature of the air relative to 
the temperature of the surface over which it moves. 
mP air carries much moisture. Over surfaces warmer 
than the air, this moisture forms cumulus clouds and 
thunderstorms; over colder surfaces, it forms stratus 
clouds and fog. In winter mP air, icing hazard presents 
a major obstacle to aviation. 

WORLD FRONTS AND FRONTAL WEATHER 

The locations of frontal zones (Figures 11 and 12), 
like the source regions of air masses, primarily depend 
upon the world pattern of temperature and the wind 
circulation of pressure systems. These, in turn, 
depend upon the climatic controls. The most vigor¬ 
ous fronts form where air masses from widely differ¬ 
ent source regions meet while still retaining their 
source characteristics—that is, where very cold air 
meets warm air. High moisture content, as in air 
over warm ocean currents, is another and highly 
significant contributor to frontal turbulence. 

Polar Front! 

Along the east coasts of North America and Asia, 
in the winter season, are concentrated the most favor¬ 
able conditions for vigorous frontal activity (Figure 
11). Here cold air masses from continental sources 
meet warm moist air from over the oceans. The 
warm ocean currents bathing these coasts greatly 
accentuate the frontal activity (pp. 11, 12). The great 
difference in temperatures of the clashing air masses, 
caused by the contrasting characteristics and close¬ 
ness of their sources, and the great amount of mois¬ 
ture that feeds into the air from the warm ocean 
currents, account for the intensity and persistency 
of these frontal zones off east coasts in winter. 

In the Atlantic the polar front of winter swings 
back and forth between the West Indies and the 
Great Lakes area, with a maximum of intensity when 
the front coincides with the coast line. Waves with 




Fig. 19. A “family” of cyclonic waves moves along a 
polar front with each outbreak of polar air 

cold and warm fronts form along this polar front 
and move northeastward along the front. Like all 
cyclonic waves, they develop low pressure centers 
along the frontal trough. They may grow into 
severe disturbances and go through the usual stages 
of development—formation, growth, occlusion and 
dissipation. 

These cyclonic waves come in “families” (Figure 
19). Each “family” of waves is associated with a 
southward surge, or outbreak, of cold polar air. 
At the beginning of a typical outbreak of polar air, 
the polar front commonly extends approximately 
through the Great Lakes area. As the polar air ad¬ 
vances, it pushes the front southward. Then the 
outbreak occurs and polar air, joining the trade 
winds, spills equatorward. 

There is no regular time interval for the large 
outbreaks of polar air but the average period between 
them is about 5*4 days. Under average conditions, 
there are from 3 to 6 cyclonic waves on the polar 
front between each outbreak of polar air. The first 
of these usually travels along a front that lies farth¬ 
est to the north. As the polar air accumulates north 


20 



of the front, the front is pushed southward and the 
last wave therefore follows a path that starts farther 
south than the path followed by the first wave. These 
“families” of polar front cyclones appear most regu¬ 
larly over the North Atlantic and North Pacific in 
the winter season. 

An understanding of the cyclone “family”-an 
understanding by pilots and by aerologists—is espe¬ 
cially important while operating over oceans when 
reports are few. That means in wartime. Pilots’ re¬ 
ports to the aerologist are frequently the only 
clue the aerologist has to storm movements. While 
these reports may not be of significance to pilots 
just returning from a mission, they may be the 
difference between success or failure of tomorrow’s 
mission. 

During the summer months the polar front of the 
Atlantic recedes to a location much farther north 
(Figure 12); and polar outbreaks, with their accom¬ 
panying “family” groupings of cyclones, are very 
irregular in summer and often do not exist at all. 

You know that, over the United States, frontal 
activity is more vigorous in winter than in summer. 
This is true because (1) polar and tropical air 
masses have greater temperature contrasts in winter, 
and (2) polar highs reach maximum developments 
then. Both of these factors lend force to the winds 
that pour into fronts. Over oceans of middle latitudes 
a third factor helps to make winter fronts more 
vigorous than summer fronts. In winter , continental 
air becomes very unstable when it moves over the 
comparatively warm ocean surface (Figure 13); in 
surmner , it remains relatively stable over the com¬ 
paratively cool ocean. Summer frontal activity (in 
middle latitudes) is therefore weak, over ocean as 



over land. The high moisture content of maritime 
air causes much cloudiness, but in the relatively 
stable summer air this moisture adds little energy 
to frontal activity. In winter, frontal activity is most 
vigorous 'over oceans off east coasts of continents, 
moderate over oceans off west coasts. Coldness of the 
winter cP air masses that move westward onto the 
oceans, and the warmth of ocean currents over which 
they move, account for the vigorous activity off east 
coasts. Modification of the air masses as they sweep 
eastward causes the modified frontal activity off 
west coasts. 

The polar front activity of the Pacific is similar 
to that of the Atlantic except that in winter there 
are usually two fronts (Figure 11). The second front 
results from the common occurrence, in winter, of 
two subtropical highs in the Pacific. (Because of the 
variable position of these highs, average wind charts 
do not show them.) When one high dominates the 
subtropical Pacific in the winter season, the Pacific 
polar front forms near the Asiatic coast, getting its 
energy from the temperature contrast between cold 
northerly monsoon winds and the tropical maritime 
air masses they meet, and from the air-warming, 
moisture-yielding Japan Current. When two highs 
occupy the subtropical Pacific, a second polar front 
extends a trough through the approximate areas 
shown in Figure 11. When this second polar front 
exists, two systems of cyclonic disturbances move 
across the Pacific. Owing to their greater sources of 
energy, however, storms that originate over the Japan 
Current and move toward the Aleutians are always 
most severe. 

During summer months the polar front of the 
Pacific, like that of the Atlantic, lies far to the 
north (Figure 12) and experiences no rhythmical 
polar outbreaks. 

Returning attention again to the winter scene in 
the Atlantic, we find that a second polar front, 
similar in nature and cause to the second polar front 
of the Pacific, sometimes—though rarely—develops. 

In winter a frontal zone appears over the Medi¬ 
terranean Sea (Figure 11). A low pressure trough re¬ 
sults from the sea surface being warmer than the land 
surface on either side, and in-flow of air from 
Europe and from northern Africa produces the 
frontal activity. Not permanent, this front is best 
developed when the clashing air masses have con¬ 
siderable contrasts. Cyclones may move into the 
Mediterranean on a path that passes either south or 
north of the Iberian Plateau. These storms, and 
those that develop on the front within the Mediter¬ 
ranean area, usually travel eastward. Some move 
northeastward into southern portions of the U.S.S.R., 


21 



while others cross Asia Minor and advance to India. 
The typical winter storms of India come from the 
west. 

The winter rains of the Mediterranean area result 
from the cyclones that develop along the Mediter¬ 
ranean front. In summer, when this front disappears, 
dry weather prevails. 

Arctic Fronts 

Where air masses from Arctic icecaps and snow 
fields meet polar or tropical air masses, Arctic fronts 
develop (Figures n and 12). These fronts are most 
vigorous where Arctic air meets warmer air over 
comparatively warm-surfaced seas. They therefore 
generally start, and attain greatest vigor, over eastern 
portions of oceans, for in high latitudes over eastern 
portions of oceans both the mP air and the ocean 
surface temperatures are warmer than farther west 
(see air temperatures, Figures 5 and 16, and ocean 
currents, Figure 10). In the North Atlantic, winter 
Arctic front activity increases when the Icelandic 
low moves farther east than normal, for this pro¬ 
duces the strongest possible contrast between the 
cold Arctic air and warm air from the southwest. 

Intertropical Fronts* 

In equatorial or tropical regions, where trade winds 
(or, in places, monsoons) from one hemisphere meet 
trade winds from the other hemisphere, weather 
fronts form. These fronts, called intertropical fronts, 
migrate seasonally with the heat equator. Figures 11 
and 12 show average seasonal locations of these 
fronts. 

The zone of intertropical fronts has in abundance 
one of the conditions conducive to strong frontal 
activity: it has much moisture in the air. It usually 
lacks, however, another condition stimulating to 
frontal activity: it lacks marked temperature contrasts 
in the clashing air masses. 

Usually the two converging air masses have a 
temperature difference of not more than 2 0 or 3 0 F. 
With much moisture in the air, however, a converg¬ 
ing of two air masses can produce severe fronts even 
when there is no apparent temperature difference 
between the two air masses. Nevertheless, in the 
tropics as elsewhere, temperature contrast retains 
merit as a front stimulator. On those occasions when 
an active outbreak of polar air from the winter 


* The intertropical fronts are hotly disputed fronts just at this 
time and are under special study. 


hemisphere reaches the intertropical front, the 
slightly colder and drier air in contact with the 
warmer and more moist equatorial air generally ; 
causes fronts of severe intensity. The strongest I 
fronts might normally be expected to develop in 
late summer of one of the hemispheres (August- 
September and February-March) when the tempera¬ 
ture differences between the two hemispheres are 
great; and when, at the same time, the zone of con¬ 
vergence between the winds from the two hemi¬ 
spheres lies farthest from the equator, causing maxi¬ 
mum wind direction contrasts between the clashing 
air masses.* # 

Intertropical frontal activity is generally weak I 
immediately following the equinoxes because then 
the two hemispheres have approximately equal tern- | 
peratures. The activity is comparatively weak when ' 
the fronts lie near the equator because there is then 
a minimum of wind direction contrast in the clash¬ 
ing air masses. Wherever the intertropical front 
remains north of the equator for most or all of 
the year, as in the Atlantic and in the central 
and eastern Pacific (Figures 11 and 12), August- 
September (late summer) is the season of greatest 
activity. In the monsoon area of the Southwest 
Pacific, the front may have quite vigorous activity 
whenever it lies at considerable distance from the 
equator. The intertropical front is seldom completely 
negligible. Everywhere and at all seasons fliers of the 
tropics know it as a major flying hazard. 

Many intertropical fronts have cold front char¬ 
acteristics. High temperatures and much moisture, 
coupled with similarity of temperature between the 
two air masses (little or no under-running of cold 
air), make these “cold front type” fronts steep and 
unstable—marked by towering walls of cumulo¬ 
nimbus. Certain fronts with warm front character¬ 
istics, however, may have gentle frontal surfaces and 
spread clouds over a wide belt. The lows that develop 
along these intertropical fronts move, in general, 
toward the west, carried by the air current of the 
trades. 

** Studying Figures n and 12, and knowing that wind deflects 
to its right in the Northern Hemisphere and to its left in the 
Southern Hemisphere, you will see that when a NE. trade wind 
crosses the equator into the Southern Hemisphere it soon be¬ 
comes a NW. trade, and when a SE. trade crosses into the 
Northern Hemisphere it becomes a SW. trade. Over tropical 
seas north of the equator in August-September, for example, an 
air mass moving from the SW. may meet an air mass moving 
from the NE. When air masses meet near the equator, on the ' 
other hand, there may be little contrast in their wind directions. 




22 








NORTH ATLANTIC 


Busiest ocean in the world for both surface craft and aircraft, the North Atlantic held such high stra¬ 
tegic importance that the “Battle of the Atlantic” demanded first attention of the United Nations. As 
aircraft assumed an ever-increasing role in winning that battle, the climate and weather of the region 
became ever-increasingly important. In the era of air transport that will follow this crisis, the air over 
the North Atlantic will become a primary link in connecting the peoples of North America with the 
peoples of Europe (Figure 20). 


This study of the climate and weather of the 
North Atlantic presents a challenge, to you—%. chal¬ 
lenge to apply aerological facts and principles (which 
you know) to an understanding of the complicated 
climate of a great ocean, an ocean over which you 
may fly on vital missions. Your success in meeting 
this challenge depends largely upon: (1) how well 
you have learned the facts and principles of aerology, 
especially as they apply to aviation; (2) how well 
you understand the climatic controls—the controls 
that make and move air masses and mold the climates 
of earth; (3) how well you can apply these facts 
and principles in developing a composite, clear pic¬ 
ture that will help you to anticipate the climatic 
conditions you will encounter in various parts of 
the North Atlantic. If you do a thorough, competent 
job in developing this climatic picture, you will be 
able to act with nature rather than against nature in 
performing your mission in this great war area. 

WEATHER HAZARDS AND HELPS TO AVIATION— 

THE GENERAL PATTERN 

Temperature and Icing 

A Relatively Mild Climate. Located between 
two large continents, both of which experience bitter 
cold winters and warm to hot summers, the North 
Atlantic has a relatively mild climate. Water heats 
and cools more slowly than land, so the oceans of 
high latitudes have more moderate seasonal tempera¬ 
ture changes than do the continents of North 
America and Eurasia. Cabot, in 1498, sailed along the 
54th parallel from England to Labrador. Except for 
floe ice and frozen shores at the Labrador coast, this 
course crosses open water the entire way—but 


Hudson Bay in the same latitude remains frozen 
half the year. The ‘northerly route over which our 
convoys move to Murmansk through unfrozen seas 
rounds northern Norway in the latitude of central 
Greenland. Hammerfest, Norway, latitude 70 °, 38', 
is the most northern ice-free port in the world; while 
Chicago’s harbor, 2000 miles farther south, freezes 
each winter. In the North Atlantic between the Brit¬ 
ish Isles and Iceland the average January tempera¬ 
ture is 40 0 warmer than is common to land areas 
in the same latitude (Figure 5). If Chicago were 
40 0 warmer in January, it would have winter tem¬ 
peratures comparable to those of Tampa, Florida. 

But temperatures of the North Atlantic are neither 
as mild nor as simple as the foregoing facts may 
suggest. Cyclonic storms with invasions of differing 
air masses bring day-to-day and week-to-week 
weather changes. And, relative to the general pattern 
of average temperatures, the western (American) 
•side of the ocean has a quite different average tem¬ 
perature pattern from the eastern (European) side. 

East Side Milder Than West Side. Figures 21 
and 22 show that, except in subtropical latitudes, 
temperatures of the eastern (European) North Atlan¬ 
tic are milder than those of the western (American) 
North Atlantic. In winter, the freezing isotherm 
(line connecting points of equal temperature) which 
runs through Boston approaches the Norwegian 
coast 1700 miles farther north. Likewise, in winter, 
the European North Atlantic has much less tempera¬ 
ture change from north to south—notice the wider 
spacing of isotherms on the European side in Fig¬ 
ure 21. The isotherms spread almost fan-like from 
the American coast. From northern Spain to north¬ 
ern Norway, a latitudinal distance of 2000 miles, 

23 





Grand 

Banns 


KEY 

Over 10,000 ft. 
5,000- 10,000 ft. 
1,000 - 5,000 ft. 
Under 1,000 ft. 


DOM/i 

REPC 


24 











































26 


Fig. 22 

































Fig. 24 


27 






























the temperature decreases only 30°; in the same 
latitudinal range in the American North Atlantic 
the temperature difference is 50°. 

Temperature contrasts between the two sides of 
the Atlantic in summer are of the same nature as 
winter contrasts, but less pronounced (Figure 22). 

Icing Levels. Temperature of the upper air holds 
great significance to the pilot because of its direct 
relation to aircraft icing. Figures 23 and 24 show the 
average level of freezing (32°F.) temperature for 
February and August. As you know, ice may form 
on aircraft with air temperature at 34°F. or 35 0 F., 
but the ranges from 33°F. to i5°F. in stratiform 
clouds, and 33°F. to o°F. in towering cumulus afford 
greatest danger. 

Figures 23 and 24 naturally resemble the maps 
showing surface temperatures (Figures 21 and 22). 
This is because temperature decreases on the average 
about 3 0 Fahrenheit per 1000 feet increase in alti¬ 
tude—it does that in non-ascending air and it does 
it when clouds form in ascending air.* On the August 
map (Figure 24), however, certain slight adjust¬ 
ments have been made to the different characteristics 
of dominant air masses. 

These maps show average conditions, indicating 
only in a general way the height below which aircraft 
can operate without danger of icing. Where the 
icing level is indicated at a few thousand feet on 

* The normal vertical lapse rate is about 3°F. per 1000 feet; 
and although the dry adiabatic lapse rate—temperature change in 
dry ascending air—is 5.5°F. per 1000 feet, the moist adiabatic lapse 
rate is about 3°F. per 1000 feet. 

ALTITUDE RANGES OF SERIOUS ICING 
IN IN 


CUMULIFORM CLOUD: STRATIFORM CLOUD' 



Fig. 25 


the winter map, invasions of fresh cP air masses may 
bring icing conditions down to the surface. Likewise, 
invasions of tropical air may send the icing level 
soaring. Owing to cyclonic activity, then—to the 
play of air masses along frontal regions—the icing 
level fluctuates from day to day, and hour to hour. 

In summer, with an average lower icing level near 
10,000 feet from Newfoundland to England, low 
flying avoids the icing hazard. Notice, however, that 
in winter over the northern part of the ocean, ice 
commonly forms at low elevations. Along much of 
the great circle air route from Newfoundland to 
England the winter icing level normally reaches 
the surface, and nowhere along this route does it 
exceed 4000 feet. High-level flying is recommended 
over this route in winter. Pilots commonly fly 
10,000 to 15,000 feet above the lower icing level , 
where low temperatures permit only rime ice to 
form. 

If we consider 33°F. to o°F. as the common 
range of serious icing in towering cumulus clouds, 
the altitudinal range of. serious icing danger in such 
clouds will be about 11,000 feet; that is, serious icing 
may be encountered from the surface to 11,000 feet, 
or from 4000 feet to 15,000 feet (Figure 25). In 
stratiform clouds, with probability of serious icing 
in the temperature range from 33°F. to i5°F. the 
altitudinal range of icing trouble is about 6000 feet 
(Figure 25). 

Northward from Newfoundland, in the Labrador 
and Greenland areas, summer presents the greatest 
icing problem. Average winter temperatures drop 
below the range of serious icing danger. 

Throughout the entire northern portion of the 
North Atlantic, spring and fall bring icing problems 
due to changeable temperature conditions associated 
with frontal disturbances. Average icing levels then 
lie intermediate between those of winter and summer. 
Low-level flying in spring and fall presents serious 
risks; frontal disturbances often necessitate high 
flying to avoid heavy glaze. 

In subtropical portions of the North Atlantic, low- 
level flying eliminates icing danger at all seasons. 


Winds 

Pressure Systems Affect the Winds. Prevailing 
winds everywhere bear close relationship to earth’s 
pressure systems. In the North Atlantic two large 
semi-permanent pressure areas (a high and a low) 
influence the winds—and the movements of air 
masses and fronts. These are (1) the subtropical 
high, called the Azores high and sometimes the 
Bermuda high because of its location with reference 


28 






















to these islands, and (2) the Icelandic low centered 
near Iceland. 

The subtropical high is primarily a warm season 
affair. Pressure begins to build up in spring when 
the subtropical ocean becomes cooler than the land 
in like latitude; by midsummer the high extends 
almost the entire width of the North Atlantic. From 
its southeastern and southern sides northeast trade 
winds blow quite steadily; and from its northwestern 
and northern sides emerges some of the air that blows 
as prevailing westerlies (Figures 8 and 9). Variable 
light winds and calms characterize the central portion 
of the subtropical high. 

In fall, as the continents cool, this oceanic high 
loses strength; by midwinter it has become quite 
weak. # Winter outbreaks of polar air from North 
America, however, repeatedly reinforce it, and the 
NE. trades blow throughout the year. 

The Icelandic low, unlike the subtropical high, is 
best developed in winter, weakest in summer, for in 


* Except at times when the Asiatic high of winter, expanding, 
pushes a tongue of high pressure westward, sometimes as far as 
Bermuda. 





Fig. 26. A storm breaks near Iceland 


winter the ocean of this area holds warmth in com¬ 
parison with land of like latitude. Pressure is not 
constantly low, because frequent outbreaks of cold 
polar or Arctic air advance across the Iceland area, 
temporarily causing high pressure. An average of 
low pressure results here, however, from the com¬ 
parative warmth of the ocean and from the frequency 
with which polar front and Arctic front cyclonic 
lows invade the region. Here lies the stormiest part 
of the North Atlantic (Figure 26). 

On the average, then, winds spiral counter¬ 
clockwise into a low near Iceland. To the south 
of Iceland, these winds are the prevailing westerlies 
of the central and northern portions of the North 
Atlantic (Figures 8 and 9). 

Surface Wind Directions. If wind blew con¬ 
stantly from the west in the northern North Atlantic 
and from the northeast in the trade wind latitude, 
weather in those regions would lose much of its 
variety and become quite monotonous. Figures 
27 and 28 show direction and velocity of surface 
winds and winds at 6000 feet. Study these wind roses. 
Through length of each arrow shaft, the roses show 
what percentage of time the wind blows from each 
direction, giving directions in eight compass points. 
Using the Beaufort scale, they indicate the average 
velocity of wind from each direction. 

On most roses all eight compass points appear. 
Nowhere do winds blow constantly from any one 
direction, neither at surface nor aloft. 


29 












on 










































32 




















20 ° 10 ‘ 




Reykjavik 


X! 



Horta 


10 * 20 ° 


30* 


^ f • ArHhangei 



50° 



ergen 







East ■' l 

England d S®*' 

• Valencia J ' 



The Hetders 

$ 



Brest 




. G' 




It^ 




SUMMER WINDS 

SURFACE and ALOFT 

For each stationz 

Lower wind rose represents 
SURFACE winds. 

Upper wind rose represents 
winds ALOFT at 6000ft. 


50 ° 


40 * 


3 <r 


20 ' 


30 c 


10 * 


Fig. 28 


33 
















34 


Fig. 30 





























The prevailing westerlies. Studying the surface 
winds from Washington and St. Johns to north¬ 
western Europe, we find westerly winds (west, 
northwest and southwest) outstandingly prominent 
| at St. Johns and St. Paul and at Arkhangelsk. At 
Washington, northerly winds prevail in winter when 
the polar front frequently lies to the south, and 
southerly winds prevail in summer when the polar 
front commonly lies to the north. Over the British 
Isles and North Sea area, westerly winds dominate 
in summer; but winter, when cyclones race across 
this region, has such extremely variable winds that 
no direction holds prominence. Yet we call these the 
prevailing westerlies. 

Bermuda, commonly covered by tropical air (Fig¬ 
ures 27 and 28), has prevailing westerly winds in 
winter and southwesterly winds in summer. 

Reykjavik, Iceland, north of the prevailing west¬ 
erlies, has the variable wind directions to be expected 
in that “gathering ground” of cyclonic storms, the 
Icelandic low. And note that the Icelandic low 
has a high percentage of calms. 

The trade winds. Within the subtropical high, as 
within the Icelandic low, variable winds prevail with 
many days of calm (see the wind rose for Horta, 
in the Azores). Winds, spiraling outward clockwise 
from the subtropical high, produce NE. trades over 
the Canary Islands; and in summer when the trades 
reach their northernmost extent they become appar¬ 
ent in the wind roses for the coastal waters west 
of Gibraltar. 

In the latitude of the Canaries the trades extend 
as a belt across the ocean. (They proved a consider¬ 
able help to Columbus.) NE. winds off the African 
coast, they become easterly winds at Miami and 
throughout most of the West Indies. 

Note the comparative persistency of the NE. 
trades, especially as shown by the wind roses of 
the Canary Island region. Frequent frontal activity 
between Bermuda and Miami lowers the reliability 
of trades in those longitudes. 

Wind Velocity. Gales frequently whip the 
ocean south of Greenland. In this area winter winds 
average about 25 knots, and in February they exceed 
28 knots more than 40% of the time (Figure 29). 
During the winter season the North Atlantic is 
seldom without a gale somewhere between New¬ 
foundland and Ireland. Gales commonly blow from 
the west, but they may come from other directions 
as well. Rapid and violent changes in wind direction, 
accompanied by blinding rain and snow, characterize 
the winter weather of this region. 


Summers are more docile. Winds, dominantly 
westerly, average slightly more than one-half the 
winter velocity. Gales are rare (Figure 30); the 
region of greatest occurrence experiences gales only 
10% of the time and coincides with that of winter 
(south of Greenland). 

Spring and fall are transitional seasons so far as 
wind velocity is concerned. Strong winds and gales 
begin early in the northern part of the region, the 
winds of fall resembling those of winter more than 
those of summer. Spring, on the other hand, is more 
like summer, with relatively few gales and with more 
navigable weather. 

Winds Aloft. The upper wind roses in Figures 
27 and 28 give a truer picture of wind conditions at 
usual levels of flight than do the surface roses. At 
6000 feet elevation over most of the stations, average 
wind directions remain about the same as at the 
surface, and equally as variable. Washington, with 
prevailing westerlies aloft throughout the year, pro¬ 
vides a slight exception, while the trade wind area 
off the Atlantic coast of North Africa provides an 
opposite type of exception, with wind aloft in these 
trades more variable than at the surface. 



35 







36 













0 20 ® 10 ° 


10 ° 


20 ® 


30 ° 


Mygg 


'buW 



y 


& 





M' 





Ahureyn $ e ytiis> 

M&i w w ° 




y/K 




'• s h0^ 

2 -^' Mgl* 


- \ 

^ 3 * 

Vy> 

iSi* £. 


-Stripy 


er< 


€Sf 



\ 

^2 





•*SilW ( 




Porrtfl 

Defgoaa 


Fia^o 


W 1 


50 ° 


m 

h?-- 

% A 

5a «^ z 

\ 

i 

Vi 

\ 


\ 

— 

S& \ 

\ 

KZ3 

0 

0 

CNJ 

10 * 

0° 


Fig. 31. Seasonal distribution of poor visibility 






















38 


Fi g- 33 































If you fly higher than 6000 feet in the belt of the 
westerlies, wind directions become more dependable. 
Above 10,000 feet, west winds blow with remarkable 
reliability. 

Over the sea, wind velocity at about 2000 feet 
normally averages 150% of surface wind velocity. 
In the westerlies, velocity generally continues to 
increase with increase in elevation. 

* 

Cloudiness and Precipitation 

The sky over the North Atlantic averages from 
five-tenths to seven-tenths cloud covered, the route 
between Newfoundland and Britain having great¬ 
est cloudiness. Winter is slightly more cloudy than 
summer. 

Between the Atlantic coast of the United States 
and northwestern Europe the dominant lower clouds 
of winter are strato-cumulus and cumulus; the sum¬ 
mer season has fewer cumulus and more stratus 
clouds. Over subtropical waters scattered cumulus 
clouds prevail at all seasons. The scattered cumuli 
cause pilots no significant difficulty, and may be 
useful in combat. Stratiform clouds, however, though 
useful in aerial attack, make patrol work difficult and 
hide bases and flat-tops. They also cause low ceilings, 
icing hazard, and precipitation that reduces visibility. 

During the winter season, precipitation in some 
form falls about 20% of the time over most of the 
central North Atlantic. Least rainy sections are the 
American coastal waters and the subtropical latitudes. 
Over northern waters much of the winter precipita¬ 
tion comes in the form of snow and cold drizzles. 
It causes both icing and low visibility; and, as if to 
increase the aggravation, gales may accompany blind¬ 
ing snow storms. 

During summer, precipitation is less widespread, 
but a large area south of Iceland has rain or drizzle 
between 10% and 10% of the time. 

Visibility 

Figure 31 shows geographical and seasonal distri¬ 
bution of poor visibility over most coastal and island 
areas of the North Atlantic. Study the graphs on 
this map. Note that each graph represents a land or 
coastal station or, in a few cases, a section of the 
ocean. The height of each vertical bar shows the 
percentage of observations at that place which reveal 
low visibility—visibility of two miles or less.* The 
higher the bar the poorer the visibility. Left-hand 


* For making Figure 31, “poor” visibility was rather arbitrarily 
defined as visibility of two miles or less; this limit was set owing 
to availability of data based on that distance. As a Navy pilot 
you’ll do much flying with visibility under two miles. 


bars indicate frequency of poor visibility (two miles 
or less) during 'winter , and right-hand bars indicate 
frequency of poor visibility during summer. 

Most coast and inland locations have more poor 
visibility in winter than in summer (Figure 31). This 
is due largely to two factors: (1) greater frequency 
of frontal disturbances in winter and (2) longer 
hours of darkness in winter. The frontal activity 
brings precipitation and frontal fog, and aggravates 
the hazard of industrial smoke. The long hours of 
winter darkness permit land fogs (and smogs* # ) to 
persist, whereas sun’s warmth in summer does much 
to clear away fog and otherwise improve visibility. 

At Newark, New Jersey, a great metropolitan 
area with much industrial smoke, visibility lowers 
to two miles or less during about 25% of observa¬ 
tions in winter and 15% of them in summer. Most 
cities of northeastern United States have slightly 
better visibility. So, also, do the coasts of Iceland, 
which have even more frontal activity than the 
U.S. but much less smoke. 

Hamburg, Germany, rivaled by other industrial 
cities of northwestern Europe, won a tall prize for 
poor visibility in winter (Figure 30). The reason: 
industrial smoke. (Allied bombing has improved the 
visibility over Hamburg!) 

Sea Fogs Worst in Summer. With smoke missing 
over the ocean (only source of industrial smoke on 
the North Atlantic: convoy funnels!), precipitation 
and fog account for most of the low visibility en¬ 
countered. Over North Atlantic water, frontal fogs 
are comparatively rare; advection sea fogs, however, 
may blanket large areas—and this sea fog can get so 
dense a seaman can’t spit through it. 


## Smoke and fog mixture—very tenacious. 



Fig. 34. Fog bank off Newfoundland (U.S. Weather 
Bureau photo) 


3 ? 







Figures 32 and 33 show that dense to moderate 
sea fogs are most frequent in summer. This is because 
polar fronts lie farthest north in summer and the 
general circulation pattern carries mT air far north 
over water colder than the air. Advection fog forms 
when surface layers of the warm moist mT air chill 
over cold seas. When strong winds blow, this fog 
may lift to become low stratus. 

Waters surrounding Newfoundland have more of 
summer sea fog than any other Atlantic area (Figure 
34). Large quantities of summer mT air move north¬ 
ward along the east coast of North America (Figure 
12), and here find extreme contrast in sea tem¬ 
peratures between the warm water of the Gulf 
Stream and cool water of the Labrador Current. Fog 
forms where the warm moist air blows over the cool 
water. European coastal seas have less summer sea 
fog because mT air moving northward over these 
waters finds less contrast in sea temperature. Figure 
31, however, shows much low visibility in summer 
(caused by sea fog) all the way from Jan Mayen 
Island to Oporto, Portugal. 

WEATHER IN THE DIFFERING AIR MASSES AND 
FRONTS OVER THE NORTH ATLANTIC 

Those parts of the North Atlantic lying on or 
near the major frontal zones, where air masses meet, 
have weather as changeable as any you’ll find. Beyond 
the range of most frontal activity, as within the 
Azores high and the quite steadily-blowing trade 
winds, weather behaves more dependably, with few 
and slight changes from day to day. To understand 
the flyability and flight problems of each part of 
the North Atlantic, you should know (in addition 
to the average weather of each area) something about 
weather’s whims. 

Both average weather and its changeability result 
from the characteristics and behavior of those 
weathermakers, the air masses. 

Three types of air masses—polar continental, tropi¬ 
cal maritime, and polar maritime—bring most North 
Atlantic weather. In addition, Arctic air influences 


- 14 , 000 ' 

- 

-12,000' 

■ 

-10,000' 

- 

-6,000' 

- 

-6,000' 

- 

- 4 , 00 qWT 

cP/*//<? cS!k - 




NORTH AMERICA IN SUMMER NORTH ATLANTIC IN SUMMER 


subpolar areas and tropical continental air affects the 
Mediterranean. Figures 11 and 12 show the sources 
of the primary air masses and the general locations 
of frontal zones along which meetings of air masses 
produce cyclonic disturbances. 

Air Mass Weather 

Polar Continental Air.* When cold or cool 
cP air from NoAh America (or cold Arctic air from 
Greenland) moves out over a comparatively warm 
ocean, its lower layers warm, become unstable, and 
develop strato-cumulus or (usually) cumulus clouds 
(Figure 13). 

Off the east coast of the United States and east¬ 
ward from Newfoundland, northwesterly winds 
often feed cP air into frontal depressions over the 



NORTH AMERICA IN WINTER LABRADOR CURRENT IN WINTER 

Fig. 36 


Atlantic. In such air streams, in the air masses back 
of the fronts, convection clouds normally develop 
very readily. In summer, when the ocean surface is 
only slightly warmer than the cP air from northern 
lands, cumulus clouds seldom build to more than 
5000 feet (Figure 35). In winter, however, cumulus 
clouds in cP air can grow tall, averaging about 
10,000 feet over the cold Labrador Current and 
about 13,000 feet over the warm Gulf Stream (Fig¬ 
ures 36 and 37).** Cumulus clouds yield showers 


* And Arctic continental air. 

## Along winter cold fronts between cP and mT air, the 
cumulus clouds may build up to 20,000 feet. 



NORTH AMERICA IN WINTER GULF STREAM IN WINTER 


40 


Fig- 35 


Fig. 37 



















of rain and sometimes hail. The taller clouds form 
“anvils”, and turbulence within the clouds may be¬ 
come excessive, but they seldom reach thunderstorm 
proportions. The ceiling averages 2500-3000 feet 
under cumulus clouds, but may lower to 1000 feet 
during showers. Except in showers, visibility remains 
good in this cP air, because no advection sea fogs 
can form in air colder than the ocean surface. Espe¬ 
cially in winter, cP air generally moves over the 
ocean at a brisk clip—often with gale force—and 
surface craft often experience their roughest weather 
in northwesterly winds from the continent. 

Anticyclones containing cP or Arctic air cause a 
different type of weather. Frequently a continental 
anticyclone centered over Canada extends south¬ 
eastward over the Grand Banks, sometimes even 
becoming joined to the Azores high. The Greenland 
anticyclone may also reach far over the ocean, and 
on other occasions a detached anticyclone, carrying 
cP air, moves onto the Atlantic. Near centers of 
these highs, with very light winds, considerable 
middle and high cloud may develop, but little low 
cloud. Further from the high pressure centers, with 
wind of Beaufort force 3 or over, strato-cumulus 
cloud becomes characteristic—it may be detached, 
covering only about two-tenths of the sky, but it fre¬ 
quently forms a continuous layer. Increased moisture 
content of the air makes this strato-cumulus more 
common over mid-ocean than near the American 
coast. The strato-cumulus ceilings vary from 1200 
feet to 4000 feet and the layer is usually quite thin.* 

Continental polar air originating over northern 
Europe sometimes visits Britain and frequently moves 
out over the Atlantic coasts of Europe. It brings 
little cloud except fair weather cumulus. The only 
handicap to aviation is haze caused by smoke from 
industrial areas. 

European cP air masses pouring southward to the 
Mediterranean (sometimes mP or mA air that has 
modified to cP over the continent) produce the bora 
winds that blow as gales down mountain slopes of 
the Dalmatian (Yugoslavian) coast and the mistral 
winds of southern France. The warm cP air of 
summer brings clear weather to the Mediterranean, 
but cold cP winter air may become unstable and 
develop thunderstorms over the warm sea. 

* Causes of this strato-cumulus cloud include (1) a slight in¬ 
stability of the lower air owing to a sea surface slightly warmer 
than the air and/or (2) the turbulence effect of wind over 
waves. The upper limit of the strato-cumulus is presumably the 
base of the subsidence inversion of the anticyclone. 


Tropical Maritime Air. Originating in the 
Azores high, some tropical maritime air flows 
equatorward as trades, some feeds poleward to 
frontal contact with polar air. Within the frontal 
zones over the North Atlantic, mT air is invariably 
met in warm sectors of the moving cyclonic depres¬ 
sions. 

The mT source region generally has excellent fly¬ 
ing weather—clear or partly cloudy and free from 
gales. Cloudiness, however, increases in mT air with 
increase in distance from the source. Cumulus and 
cumulo-nimbus clouds become increasingly numer¬ 
ous to the south and west, and stratus clouds prevail 
to the north and east. Movement of mT air over 
warmer water (warmer than the air) causes cumuli- 
form clouds (Figure 17); movement over colder 
water causes stratiform clouds (Figure 15). The scat¬ 
tered cumuli that develop as this type of air moves 
over warmer water cause only minor flying troubles. 
The stratus clouds, however, can be a great nuisance. 

Stratus overcast often forms in the mT air in 
warm sectors of cyclonic depressions moving east 
or northeast across the Atlantic. Drizzle falls. Low 
ceilings, usually below 800 feet, may drop to 400 or 
300 feet. They sometimes reach the surface, produc¬ 
ing fog, frequently in patches. 

Even before mT air reaches the polar front zone, 
stratus or fog may form—especially in summer when 
the polar front lies far to the north. Most of the 
summer fog of the Newfoundland area and off the 
New England coast forms in mT air that chills as 
it moves over cold water (Figure 38), and much of 
the fog of the approaches to the English Channel is 
of the same origin. When strong winds blow, the fog 
generally lifts to low stratus.* 

Polar Maritime Air. This is polar continental 
air turned maritime; so, like a landsman turned sea- 


* With a pronounced inversion present near sea level, the 
fog may fill the entire space below the inversion. Such fogs are 
much more persistent than radiation fogs, and strong winds may 
have little dissipating effect. 


mT air ^ 

<=========£> 

SEA FO6 

WARM GULF STREAM 

COLD LABRADOR CURRENT 


Fig. 38 


41 











man, it may be in any stage of transition between a 
“landlubber” and a true “salt”. When air from 
Canada, for example, has moved hundreds of miles 
out over the North Atlantic we may call it mP, 
although it still brings weather much like that de¬ 
scribed for cP air over the North Atlantic. When 
the air has had a comparatively short sea passage, 
cumulus clouds (with some cumulo-nimbus) char¬ 
acterize the weather; but after a longer stay over 
the ocean, clouds become strato-cumulus. 

This is the principal type of air mass reaching 
Britain and the North Sea area. Having crossed the 
ocean, it has picked up much moisture. In winter 
(and often in fall and spring) it contributes heavily 
to frontal precipitation. In summer (and sometimes 
in fall and spring) it may yield convection showers 
or even thunderstorms over warm European land. 

Sometimes mP air follows a southerly path across 
the Atlantic and then moves northeastward to the 
seas around Britain and off Norway. Having grown 
quite warm and moist, it may be chilled over colder 
seas. This produces weather similar to that in mT 
air moving northward. Stratus develops, yields driz¬ 
zle, and sometimes lowers to fog. Reaching European 
coasts, it brings low ceilings and rain over cold 
winter land and develops cumulus clouds and showers 
over warm summer land. 

Arctic Maritime Air. Pouring southward from 
the Arctic between Greenland and Norway, this 

42 


air can grow unstable over the comparatively warm 
waters of the European North Atlantic. Towering 
cumulus may form—with showers of rain, snow, or 
sleet. 

Tropical Continental Air. Clear and excellent 
flying weather, except for dust haze, characterizes 
the cT air common over the Mediterranean. 

Frontal Weather 

As in the “home skies” over the United States, 
so in the air over oceans, frontal activity causes 
much of the difficult flying weather. You should 
know the locations of the principal frontal zones of 
the North Atlantic (Figures n and 12) and why 
frontal activity is most common in those areas 
(pp. 20, 21). 

Locations of Fronts. In winter, polar air from 
North America meets mT air along a front that 
may be anywhere from the Great Lakes to the West 
Indies. Activity along this polar front comes in 
cycles. A cycle starts with the front well north, and 
surges of polar air push the front southeastward. 
When the front arrives over the Gulf Stream its 
activity becomes most intense, stimulated by maxi¬ 
mum temperature contrasts in the cP and mT air 
masses and by instability of the cP air mass acquired 
when its lower layers warm and absorb moisture 
over the Gulf Stream. As the cP air surges still 
further southeastward, it becomes so like mT air that 
the front weakens and dissipates. The air of cP 
origin has become part of the subtropical high. A 
polar outbreak has culminated. A new advance of 





North American air then begins (a new cycle com¬ 
mences), with the new polar front forming far to 
the northward. 

A cycle of polar outbreak along this front re¬ 
quires, on the average, about 5 to 8 days. During 
each cycle about 3 to 8 waves (a “family” of cy¬ 
clones) develop along the front and advance north¬ 
eastward. Because the front keeps moving southeast¬ 
ward, each wave member of a cyclone “family” 
starts further south than its predecessor and follows 
a more southerly track (Figure 19). In the American 
Atlantic the cyclonic lows move forward at an 
average speed of 700 miles a day—about 25 to 30 
m.p.h. as compared to an average speed of 40 to 55 
m.p.h. for cyclones over the United States. They 
slow down in midocean; and in the area of the 
Icelandic low the younger members of each “family” 
overtake the first “big brother” and thus merge with 
and accentuate the Icelandic low. 

Outbreaks of polar air, forcing the Atlantic polar 
front southward, occur chiefly in winter with some 
in fall and spring. The polar front of summer lies 
northward—with the average summer storm track 
extending from the St. Lawrence Valley, across 
Newfoundland and on toward Iceland (Figure 12). 

Arctic air, meeting mP air, stimulates frontal 
activity from south of Greenland eastward (Figures 
11 and 12). Storms that move along this Arctic front 
either pass north of Europe or may strike the Euro¬ 
pean coast anywhere from Norway to Britain and 
France. Clashes between mT air and mP air also pro¬ 
duce fronts along which cyclonic storms move from 
the central North Atlantic to the coast of Europe. As 
on the American coast, so in western Europe, cy¬ 
clonic activity is most frequent and most intense 
in the winter season. Then, also, it extends farthest 
south. Because polar and Arctic air masses penetrate 
farthest south in winter, the Mediterranean enjoys 
practically front-free summer and has most of its 
frontal activity and stormy weather in winter 
(Figures n and 12). 

Characteristics of Warm Fronts. Over oceans, 
as over land, fronts do not always fit a standardized 
pattern; for frontal weather depends upon the 
characteristics of the air masses involved. In the case 
of warm fronts over the North Atlantic the warm 
ascending air is almost invariably mT. Generally it 
is stable, producing stratiform clouds; rarely it is 
conditionally unstable and builds towering cumulus 
above the frontal surface. 

Continuous light rain or drizzle (with showers, 
if the overriding air is conditionally unstable) usually 
commences some distance ahead of the frontal pass¬ 


age—125 miles to 250 miles in advance of the surface 
front. This frontal precipitation ceases soon after the 
warm front passes, though the warm sector of the 
cyclone often has intermittent rain or drizzle. 

Precipitation falling through the cool air ahead of 
the warm front causes formation of a low stratus 
cloud that usually lowers ceilings to 500 or 1000 
feet throughout a 100 mile belt preceding the warm 
front. During passage of the front the ceiling seldom 
drops below 500 feet. Frontal fogs are rare. 

Characteristics of Cold Fronts. When a winter 
cold front, advancing from the United States, arrives 
over warm waters of the Gulf Stream, its activity 
grows more vigorous. Two reasons are: (1) warming 
of the surface layers make the cold air increasingly 
unstable, and (2) moisture absorbed into the air 
adds energy—the latent energy of condensation— 
that greatly stimulates frontal activity. 

The high moisture content common in maritime 
air might be expected to make fronts over oceans 
more intense than those over land;* and this is usually 
true (that is, severe fronts develop over oceans) 
when the clashing air masses, in addition to contain¬ 
ing high moisture content, have strong temperature 
contrasts. When a cold front has moved over hun¬ 
dreds of miles of ocean, however, the temperature 
of the advancing air mass becomes modified so that 
the advancing air is not much colder than the air 
it displaces. This makes most midocean cold fronts 
mild. 

Cold fronts over the North Atlantic, then, may 
vary from turbulent squall lines (as common in 
winter over relatively warm water off cold conti¬ 
nental coasts) to extremely mild frontal activity, 
sometimes even without precipitation. In the most 
commonly experienced cold fronts, a nine-tenths to 
ten-tenths cover of tall cumulus or cumulo-nimbus 
clouds towers along the wind shift line, with ceilings 
generally 700 feet to 1300 feet. Most of the weather 
is confined to a zone 70 to 100 miles ahead of the 
surface wind shift. After passage of the front, clouds 
in the advancing air mass become strato-cumulus or 
scattered cumulus (Figure 40). 

Characteristics of Occluded Fronts. Because 
occluded fronts over the North Atlantic always 
separate two maritime polar air masses (and mP air 
masses have a great range of possible characteristics), 
weather conditions at the passage of occluded fronts 
have a great range of possibilities. As in fronts over 

* Also, most cold fronts over the Atlantic have a blunt nose. 
This causes, at the front or a short distance ahead of it, more 
severe conditions than might otherwise be expected. 


43 




Fig. 40. Flying conditions in a representative winter cold front over the temperate North Atlantic 


land, the most severe frontal conditions are usually 
found just north of the low-pressure center in the 
initial stages of occlusion. 

Warm-type occlusions frequently resemble warm 
fronts, but with more probability of convection 
cloud in the uplifted warm air, and consequent 
showers. The rain belt accompanying warm occlu¬ 
sions is from 100 to 250 miles wide, extending 25 to 
50 miles behind the surface occlusion and much 
farther ahead. An overcast sky frequently lowers 
the ceiling to 500 or 1000 feet. 

Cold-type occlusions (Figure 41) almost invari¬ 
ably have cumuliform clouds and bring showers. 
Drizzle often falls from the stratiform clouds in the 
warmer air preceding the occluded front; cumulo¬ 
nimbus clouds along the front produce heavy show¬ 
ers; and lighter showers from strato-cumulus or 


scattered cumulus prevail in the colder air following 
the front. Ceilings are higher under this type of 
occlusion—usually 1000 to 1500 feet. 

FLYING WEATHER OF SPECIAL AREAS 

The Navy pilot may fly anywhere, but some 
patrol and transport and combat areas hold more 
interest and importance than others. The following 
paragraphs treat certain special areas of the North 
Atlantic, emphasizing weather phenomena most 
pertinent to your job as a Navy pilot. 

From North American Shores to Iceland 

The northwestern North Atlantic, from New¬ 
foundland and Labrador to Iceland, may be consid¬ 
ered a unit from the operational standpoint. A part 
of the Western Hemisphere for which the United 
States has certain responsibilities, these are home 



Fig. 41. Flying conditions in a representative winter cold front occlusion over the temperate North Atlantic 


44 















































coasts and home waters. You may patrol off New¬ 
foundland or Greenland or cross this region while 
on convoy or air transport duty along the shortest 
route to Europe. 

The pilot must recognize two sorts of weather 
limitations: (i) weather that limits his getting up 
to or down from his flight level (terminal weather); 
and (2) weather that limits his continued flight after 
gaining desired altitude (enroute weather). 

In the first case, terminal weather, many condi¬ 
tions may restrict operations: fog, poor visibility due 
to precipitation, low ceiling, adverse winds, or severe 
icing conditions near the surface. One of these con¬ 
ditions or a combination of them may be spread over 
large areas. On some occasions, however, terminal 
weather hazards may be extremely localized, with 
one base closed to take-off and landing while a 
nearby base enjoys hazard-free weather. Fog, in 
particular, may be localized; it often shrouds New¬ 
foundland coasts in summer while the interior re¬ 
mains fog-free, and vice versa in winter; and it 
seldom hides all Iceland coasts simultaneously. 
Winter hours of daylight in Greenland and Iceland 
are very limited. Particularly at the Greenland bases 
tucked into deep fjords, there are periods when the 
sun does not rise high enough to top the sides. A 
weird twilight results. 

En route weather imposes few limitations on high 
flyers. In winter, flight at 20,000 feet tops both icing 
temperatures and most clouds (Figures 40 and 41); 
and in summer, flight at that level tops almost all 
clouds. Westbound flights at 20,000 feet, however, 
buck a strong headwind; also, such missions as patrol 
duty do not permit high altitude flying. Flight under 
the clouds encounters much frontal precipitation 
with low ceilings and ice in winter and much fog 
in summer. The pilot must consider en route weather 
in terms of his mission and his aircraft. 

In winter, flying weather in this part of the North 
Atlantic is much more severe than continental 
weather because of heavier icing, higher cloud tops, 
and more frequent gales. 

Maritime air carries much moisture, making icing 
by far the greatest hazard of winter. The icing level 
remains at or near the surface much of the time 
(Figure 23), leaving insufficient clearance for safety 
in low flying. Loaded planes taking off in above¬ 
freezing temperatures may encounter heavy icing 
on climbing through a cloud layer that may be 
5000 to 10,000 feet thick. 

In spring and fall, flying conditions are frequently 
worse than those of winter, because frontal activity 
becomes erratic and sometimes more intense during 
these transitional seasons. 


In summer, flight weather over the ocean is much 
milder than over the continent. Stable conditions 
predominate over these cool northern waters, for the 
ocean is cooler than the summer continent and north¬ 
ern waters are cooler than subtropical waters from 
over which much of the air comes. Even the frontal 
zones have slight turbulence, and stratus and strato- 
cumulus clouds prevail. The Icelandic low is weakest 
at this season, frontal activity is weak and infrequent, 
and winds seldom hinder operations. Icing is no great 
problem in summer. Instead, however, sea fogs 
become a persistent nuisance (Figure 33). 

Newfoundland. Storms frequent Newfoundland 
skies. Winter storms are worse than summer storms. 
When the polar front of winter lies over this area, 
the cyclonic storms that move along the front come 
out of eastern United States. Owing to temperature 
contrasts in the clashing air masses of winter, and 
because the storms pick up much moisture southwest 
of Newfoundland, they bring this area intense 
activity and severe weather changes. Summer cy¬ 
clones, moving more directly from the continent and 
involving air masses with less temperature contrast, 
are more mild. During the hurricane season, chiefly 
July through October, cyclones of tropical origin 
(often uniting with polar front storms) sometimes 
pass over or near Newfoundland. They have lost 
much of their original intensity before reaching so 
far north, but can cause high seas and can restrict 
aviation. 

Air mass weather may also bring flight problems. 
The cP air masses direct from North America bring 
dominently fair weather; the moisture laden maritime 
air masses cause the flight problems: 

During summer, southerly winds bring mT air 
masses that produce the dense sea fogs of that season 
(Figure 38)—sea fogs that may lift to low stratus 



45 




when wind reaches several knots. Fortunately, even 
with thick fogs off-coast, coastal strips may remain 
clear. 

During winter months, easterly or northeasterly 
winds bring mP air. Moving over the cold Labrador 
Current and onto Newfoundland, cooling from 
below, this air develops a low cloud deck that often 
produces a freezing drizzle. Look out for icing, for 


Good Flying 'on top” 


mP air ot winter moving 
from the east or northeast 


6000ft. __—-_ v 

4cing in clo ud ondj n - . ^- 


Newfoundland in winter 


Labrador Current in winter 


Fig. 42. Flying conditions in mP winter air moving from 
the Labrador Current onto Newfoundland 


low visibility, and for low ceilings. These conditions 
can last for several days. Uplift of the air over coastal 
hills merely lowers the ceiling, thickens the cloud 
deck, and increases the icing. But flight is good above 
the cloud layer (Figure 42). 

Labrador. The best winter flight routes of the 
northwestern Atlantic lie north of icing troubles. 
From Labrador to Greenland, in winter, tempera¬ 
tures are generally too cold for aircraft icing (Figure 
23). Labrador also lies north of the principal storm 
tracks (Figure 11). cP air dominates, and brings 
much clear weather. 

Weather advances poleward for the summer, 
bringing to Labrador some cyclonic activity and 
icing troubles. 

Greenland. Because cyclonic storms of both 
winter and summer converge toward the Icelandic 
low, southern Greenland has frontal disturbances— 
more than Labrador, though far fewer than New¬ 
foundland or Iceland. 

Flying hazards associated with fronts include low 
ceilings, poor visibility caused by precipitation, and 
icing (more prevalent in summer). The two greatest 
air mass weather hazards are (1) fog (common in 
summer at places exposed to the sea, while inland 
places have most fog in winter), and (2) gusty fall 
winds produced by cold and heavy Arctic air 
tumbling off the Greenland icecap. 

Local coastal features have an important bearing 
on Greenland weather. Summer heating is confined 
to narrow ice-free tracks of land along the coast; 
and the coast has many fjords and jutting headlands. 
One nook of land may be protected while even a 
light summer breeze sends fog across an exposed strip 
nearby. Strong fall winds may sweep a valley while 
calm air nestles in neighboring headland nooks. 

46 


Hence the difference in weather between neighbor¬ 
ing places. Somewhere a base lies open! 

Shores of Northwestern Europe 

The principal movement of air over northwestern 
European shores is from the west; and since air from 
this direction has a long history over water, the 
climate of northwest Europe is maritime. That is, it 
has moderate temperatures, frequent low clouds and 
fog, periods of poor visibility, and more precipitation 
than a pilot needs (Figure 43). 

The air flow from the west is most constant in 
summer, when the Azores high is best developed 
and low pressure often covers much of continental 
Europe. During winter, the Azores high is weaker 
and relatively high pressure prevails over Europe; 
so in winter, to a greater extent than in summer, 
continental air occasionally reaches the European 
shores and seas. It is also during winter that, owing 
to the development of the Icelandic low, air masses 
frequently move to European shores from a southerly 



Fig. 43. More precipitation than the pilot needs 
(U.S. Navy photo) 


direction. In winter, then, with a greater clash of 
air masses than in summer, cyclonic storms are more 
frequent. 

General Flying Weather. Cloudiness and ceil¬ 
ings. Most portions of this area have a considerable 
amount of cloudiness at all seasons. During January 
the average cloudiness over practically the entire 
region is greater than seven-tenths; and July has an 
average of between six- and seven-tenths sky cover. 
Californians would call this a region of “filtered 
sunshine”. 

Not all regions are equally cloudy. For example, 
windward coasts of Scotland and Norway average 







about eight-tenths while, to the leeward, eastern 
England and the interior of Sweden are somewhat 
less cloudy, averaging about a six-tenths cloud cover. 

May is generally the clearest month. Dry northerly 
winds from an anticyclone which develops in late 
spring over Scandinavia account for spring being 
the clearest and driest season over much of north¬ 
western Europe. At some places a second period 
of minimum cloudiness occurs in September, after 
the season of maximum convection and before the 
winter cyclones become more active. 

From October to May the ceiling is usually an 
overcast of stratus clouds, while during the summer 
broken skies are more frequent. Clear skies are rare 
at any hour or any season. Under stratus conditions, 
ceilings are generally below 2000 feet, especially in 
early morning and during winter months. During 
winter, ceilings average less than 1000 feet for 3 to 
10 days a month. 

Fog and visibility. The distribution of fog and the 
range of visibility follow the general pattern and 
seasonal distribution of cloud amount. 

Winter and autumn are the seasons having the 
most fog (Figure 32). Cold season fogs are chiefly 
of radiation type. This is a land fog which forms as 



Fig. 44. Formation of radiation land fog, by night cooling 
in anticyclonic weather 



Fig. 45. Formation of advection land fog by drift of warm air 
over cold ground 

a result of radiational cooling at night under condi¬ 
tions of quiet anticyclonic weather, high humidity, 
low wind movement and clear sky. Since such 
conditions also favor the accumulation of smoke 
in the lower atmosphere, the winter radiation fogs 
are greatly intensified in manufacturing areas by 
soot and smoke. North Sea coasts of Belgium, Hol¬ 
land and Germany (and also Scotland, Denmark and 
southern Sweden), are extremely foggy in winter- 
fog being reported on one-third or more of the 
winter days. Figure 44 shows an example of radia¬ 
tion fog. 

Some winter fogs, however, result from the chill¬ 
ing of warm moist maritime air as it drifts over cold 
land. Figure 45 shows an example of this advection 
type of winter fog. 

Summer fog is, in general, a coastal phenomenon 
produced when stable mT air moves over colder 
water. It occurs most frequently in the English 
Channel when the warm sector of an extratropical 
cyclone covers the area. 

The diurnal variation in summer fog is note¬ 
worthy. At night and in early morning, the coasts 
are comparatively clear; but you’ll see banks of fog 
in the offing. These banks creep in with the day¬ 
time sea breeze. Toward early afternoon, visibility 

47 



























on the coast may lessen noticeably; but it improves 
again toward evening as the sea breeze dies away. 
These fogs penetrate inland only a mile or two and 
are not more than 2000 feet deep. 

In general, visibility is best in spring and summer 
and poorest in winter, best in late afternoon and 
poorest in early morning.* 

Another factor should be mentioned in this area 
of poor visibility. That is, the difference between 
horizontal visibility (which is usually tabulated) and 
vertical, or air to ground, visibility. In some cases, 
surface visibility may be good when upper haze 


PLANE 



layers, not noticeable from below, effectively obscure 
the ground when viewed at a flat (oblique) angle 
from above. In other cases, when there is a shallow 
layer of mist or fog, a pilot may experience no diffi¬ 
culty in contact flying, for the ground may be 
clearly discernible when viewed from above; but 
when you come in to land, the aerodome may be 
lost from view (Figure 46). 

Icing. The largest amount of cloudiness and pre¬ 
cipitation, with winter temperatures near the freez¬ 
ing point, cause much icing danger over European 
Atlantic shores during the cold season. Figure 47 
gives the average height, in thousands of feet, of the 
icing zone (32°F. to 8°F. is used as the icing zone in 
this illustration). (Note: The higher summer and 
autumn icing level of northern Scandinavia over 
southern Scandinavia may possibly be due to the 
long hours of summer sunlight.) 

Surface temperatures over all the area except Nor¬ 
way and the Baltic region average above freezing 
(Figure 21), thus icing on the ground is rare. Over 
the North Sea the normal height of the icing level 


* The most frequently reported range of visibility is two to 
ten miles, but it should be emphasized that most of the observa¬ 
tions summarized were made before the war and are likely to 
indicate better conditions than those which you are likely to 
encounter. Industrial plants are working harder now, and also an 
effort is being made to produce as much smoke as possible in 
order to screen important objectives. 


in winter is about 3000 feet (Figure 23); tempera¬ 
tures too cold for severe icing are found above about 
9000 feet in stratiform cloud, 14,000 feet in cumuli- 
form cloud. 

Winds and gales. Surface winds are mainly west¬ 
erly or southwesterly (Figures 8 and 9)—except along 
the western coast of Norway where southerly winds 
prevail (because western Norway lies along the east¬ 
ern edge of the Icelandic low). Wind directions , 
however , are quite variable due to frequent cyclonic 
disturbances (Figures 27 and 28). 

The British Isles and coastal areas of Norway, 
unprotected against the westerly winds of the North 
Atlantic, have relatively high wind velocities—8 to 
20 knots in winter, 5 to 15 knots in summer. The 
average wind speeds at all stations average from 5 
to 15 knots. 

Winds of gale force blow mainly during winter 
months and along the western coasts. Blustery Edin¬ 
burgh averages 30 gales per year; the sheltered coast 
at the head of the Gulf of Bothnia averages only 0.7. 

Winds Aloft. The upper air circulation is quite 
similar to that at the surface (Figures 27 and 28). 
Westerly to southwesterly winds prevail during all 
season in those areas south of Latitude 60 °N, 
Over Norway, the winds aloft are mainly southerly 
below 10,000 feet, becoming west to north above 
that level. The westerly winds aloft are most domi¬ 
nant during the autumn, at which time the highest 
velocities are also recorded (15 to 30 knots); while 
the most variable winds aloft with the lowest mean 
velocities occur during the spring (12 to 25 knots). 

Air Mass Weather. You will find it useful to 
know what type of flying weather you can expect 
in each of the different air masses that visit north¬ 
western European shores. 

Arctic maritime. mA air, originating in Green¬ 
land or over the Arctic Ocean east of Greenland, 
may come to northwestern Europe in all seasons 
except summer. It brings cold waves that can persist 
for days. This air becomes unstable over the rela¬ 
tively warm seas and develops towering cumulus or 
cumulo-nimbus clouds, with showery weather— 
usually snow showers in winter and hail or snow 
showers in spring and fall. You’ll find that winds are 
northerly at 20 to 45 knots, and turbulence is severe 
in the clouds and over rough country. Visibility is 
good, except in the showers. 

Arctic commental. cA air, carried by east or 
northeast winds from the interior of northern Europe, 
brings a less stormy type of cold spell. The sky may 





be clear, or a light snowfall may come from a layer 
of strato-cumulus. Normally visibility is good (12 
to 30 miles); but at night and in early morning, 
smoke hangs low and blots out industrial cities. The 
north to east winds average 20 to 40 knots. The air 
mass is shallow and above about 3000 feet you’ll find 
westerly winds. 


Tropical maritime . mT air, a warm, moist south¬ 
westerly flow from the subtropical high of the 
Atlantic, will give you even fewer bumps than the 
polar maritime that approaches Britain from the 
southwest. But it will give you even more low ceil¬ 
ings and fogs, and probably a drizzle from the low 
stratus. Visibility averages less than 2/2 miles. 


Volar continental. cP air resembles the Arctic 
continental. 

Volar jnaritime. mP air may reach northwestern 
Europe from either northwest or southwest. Coming 
from the northwest, it is colder than the underlying 
surface, resembling Arctic maritime air. It becomes 
turbulent, forms cumulus or cumulo-nimbus clouds, 
and develops instability showers. This is the usual air 
mass of this region in summer, and it is a frequent 
visitor in winter. Flying conditions are fair, but 
avoid the thunderstorms. 

When it comes from the southwest, polar mari¬ 
time air is warmer than the underlying surface. This 
air won’t be bumpy, for it becomes stable over the 
cool surfaces. Thick, dark stratus clouds develop, 
sometimes yielding steady precipitation; and fogs are 
common, especially along coasts. Visibility is usually 
less than three miles. 

20 


Tropical continental. cT air, blowing in sum¬ 
mer from southern Europe, brings clear weather to 
Britain, spotty showers to Norway. The air may 
be gusty and slightly dusty, but you’ll fly contact. 

The Mediterranean 

The Mediterranean Sea and adjacent coastal lands 
have a characteristic climate so distinct and well 
known that the name Mediterranean climate is 
applied to this type in all parts of the world. “Med¬ 
iterranean” climate is found in southern California, 
central Chile, the Cape area of South Africa, and 
parts of southern Australia. These various sections 
of the world have small climatic differences, but 
great similarities. 

Regions with “Mediterranean” climate are all on 
the west coasts of continents (or exposed to maritime 
air from the west), and in the same latitudes (30° 

20 



Fig. 47. Average height of icing zone 


49 



















































to 45 0 north or south of the equator). Areas in 
these latitudes feel the seasonal shifts of the pressure 
and wind belts—they are under the influence of the 
trade winds and subtropical highs during the sum¬ 
mer, and under the influence of the westerlies and 
polar (or Mediterranean) fronts during the winter 
(Figures n and 12). The climate is therefore char¬ 
acterized by: 

(1) Dry summers and moderately moist winters. 

(2) Warm to hot summers and mild winters. 

(3) A high percentage of sunshine, especially in 
summer. 

Air Masses of the Mediterranean Sea and 
Shores. Arctic maritime and polar maritime. mA 
and mP air sometimes moves southward across 
France in the rear of strong cyclones over western 
Europe, and penetrates to the Mediterranean. These 
air masses become unstable as they move south and 
their instability increases over the warm Mediter¬ 
ranean. In winter this causes heavy showers and 
thunderstorms—more intense in mA than in mP air. 
In summer mA air is absent; and the mP air, becom¬ 
ing warm and relatively dry during its passage over 
the heated lands to the northwest, gives good flying 
weather over the Mediterranean. 

Arctic continental and polar continental (cA and 
cP ) air. In winter, continental air from northern 
Europe often moves southwestward around the west 
side of the great Asiatic high, reaching the Mediter¬ 
ranean as a very cold and dry air stream. Some¬ 
times it is an originally maritime air mass which 
has become dried by traveling across Europe. On 
reaching the warm Mediterranean, the air mass 
acquires moisture and becomes unstable by being 
heated in the lower layer. This causes cumulus and 
cumulo-nimbus clouds, and showers. 

Especially strong outbursts of Arctic air some¬ 
times develop behind a cold front, or when a strong 
high exists over Europe with a low in the Mediter¬ 
ranean. These outbursts produce cold winds that 
often blow as gales through the valleys that break 
the mountain chain of southern Europe. Mistral is 
the name given to these strong winds in the Rhone 
Valley. The mistral may reach a velocity of 80 to 
85 knots over the Rhone mouth and sometimes 
extends to Malta and North Africa. In the Adriatic 
Sea and in the Black Sea similar winds are known as 
bora. Violent gusts and squalls sometimes reach 100 
knots on the eastern side of the Adriatic (Dalmatian 
Coast). 

In summer, cP air arrives from a heated continent. 
Passing over the cooler sea it becomes stabilized and 

50 


gives the fine weather characteristic of the eastern 
Mediterranean summers. 

Tropical maritime (mT) air. From its source 
region in the Azores high, mT air enters the Med¬ 
iterranean from the west. In winter, this mT air 
sometimes overrides mP air, forming depressions 
that move eastward along the Mediterranean front, 
and yielding rain from stratiform clouds. In sum¬ 
mer, the Mediterranean is warmer than the neigh¬ 
boring parts of the Atlantic, so mT air arrives colder 
than the sea surface. Fair weather cumulus clouds 
form, but no rain results. In contrast to conditions 
in the North Atlantic, visibility is generally good 
when mT air invades the Mediterranean. 

Tropical continental (cT) air. Clear and excel¬ 
lent flying weather, except for dust haze, charac¬ 
terize cT air. 

In winter, the chief source is North Africa. It 
often moves to the Mediterranean as a southerly 
wind in advance of a depression over the western 
part of the Mediterranean. Warm and dry in its 
source region, it picks up moisture over the sea and 
arrives at Malta and the northern shores as a humid 
and oppressive wind called the sirocco. 

In summer, cT air surrounds the Mediterranean. 
It is very warm and exceedingly dry and becomes 
stable when it moves over the comparatively cool 
sea. If the winds are strong, the water vapor 
acquired over the sea is diffused rapidly upward, so 
that the air retains its dry dusty characteristics far 
out to sea. If winds are light, little mixing takes 
place, and the lower layers become humid and 
oppressive. 

General Flying Weather. You can fly without 
weather restrictions on 25 to 29 days each month 
during summer (May to September). In winter, 
weather imposes some restrictions on all except 8 to 
10 days a month, but most winter days have sev¬ 
eral hours of good flying weather. 

Precipitation. Most rain falls during the winter 
season with the passage of extratropical cyclones. 
Summer is dry. And most rainfall occurs as showers 
rather than as steady rain. The amount of rain is 
considerably influenced by local topographic con¬ 
ditions, and generally diminishes from north to 
south. The number of rainy days is very moderate 
even during the so-called wet season; no month has 
more than about 15 days with rain. In winter, part 
of the precipitation of mountain areas falls in the 
form of snow. Snow is of course more frequent in 
the north than in the south, but occasional snowfalls 
occur in the coastal areas of northwest Africa. 


Thunderstorms. The number of thunderstorms 
decreases generally from north to south and from 
west to east (Europe is more thundery than Africa, 
and Spain is more thundery than Greece). Southern 
France, northern Italy and the Balkans have about 
20 to 30 thunderstorms a year, while southern Italy 
has only 10 to 15. On the African coast, Tunis has 
the most thunderstorms, with summer the most 
thundery season. The rest of the African coast has 
almost no thunderstorms in summer, a maximum 
number in autumn, and some in winter. Palestine 
and Syria record only 10 days with thunderstorms, 
and Egypt has 5 or less. 

Cloudiness and ceilings. Like rainfall, cloudiness 
decreases from north to south. The Egyptian desert 
area has scarcely any cloudiness, while some stations 
in the mountainous Balkans have seven-tenths cloudi¬ 
ness in midwinter. Most of the Mediterranean area 
records five-tenths cloudiness in winter and two- 
tenths in summer. Yet even the winter* skies are sel¬ 
dom overcast for a whole day—cloudiness resulting in 
rain which lasts only a few hours alternates with 
periods of clear skies. Unlimited ceilings prevail in 
summer; and winter ceilings drop low only under 
thunderstorms, along fronts, and on clouded moun¬ 
tain slopes. 

Visibility. Mountain peaks and interior valleys, 
particularly in the Balkans, may have 10 to 15 days 
with fog each month during the winter; but fog is 
rare at sea level and almost never occurs in the 
southern and eastern Mediterranean. Offshore haze, 
on 10 or 20 days a month in summer, cuts visibility 
to below 6 miles in the western Mediterranean; and 
the dust-burdened winds from the Sahara occa¬ 
sionally blur targets so that they cannot be seen 
from high altitudes. In winter, frontal storms lower 
visibility over the open sea for short periods on two 
or three days a month. In general, when flying in 
the Mediterranean you’ll see far and well. 

Icing. Icing may be experienced in clouds at 
levels above 2500 feet in cold waves of winter, and 
it may be a slight danger above 11,000 feet in 
summer. 

WEATHER MAPS AND FLIGHTS OF THE NORTH ATLANTIC 
Map Series Illustrating Frontal Sequence 

From your study of continental weather over the 
United States, you know that weather moves and 
changes from day to day. Atlantic Ocean weather 
also moves and changes. A study of climate or 
weather history must therefore include considera¬ 
tion of this day-to-day parade of the skies. No two 
weather situations are exactly alike. Certain gen¬ 


eral principles concerning weather movement and 
changes can be learned, however, from the study 
of a series of consecutive weather maps. 

You know that ocean weather reports, scant even 
under normal conditions, arc; almost entirely missing 
during wartime. Weather maps for ocean areas are 
therefore based upon a few scattered reports, and 
upon knowledge of the past history of the forma¬ 
tions and the changes that can be expected to occur 
as the weather advances. 

In studying ocean weather maps you must keep 
in mind the life history of cyclones from the devel¬ 
opment of waves through occlusion and dissipation. 

The accompanying series of weather maps (Fig¬ 
ures 48-56) presents polar front activity over the 
North Atlantic during nine days of February, 1940. 

The weather map, Figure 48, shows a series of 
cyclones with their accompanying frontal activity. 
The low over the continent is a well-developed 
cyclonic area with a warm front and two cold 
fronts. The low over the Western Atlantic shows 
the cold front moving up toward the warm front 
and occlusion almost ready to take place. The low 
over the eastern Atlantic has already occluded. 
Three cells of high pressure are worthy of note on 
this map; viz., the subtropical high in the southern 
part of the North Atlantic, which is a permanent 
high but migrates north and south with the season; 
the high pressure over Iceland, which has displaced 
an area of semi-permanent low pressure usually 
found in this area; and the area of high pressure 
which is flowing down across the North American 
continent. 

A flight from Bermuda to the Azores is also 
shown. This flight started at 2100, nine hours after 
the drawing of this map, during which time the for¬ 
mations would have moved and changed. This flight 
was made at about 9000 feet on top of the cloud 
formation. At 0300, the pilot reported that he was 
flying over strato-cumulus clouds covering eight- 
tenths of the sky. This apparently was the cold front. 
During the 15 hours that had passed since the draw¬ 
ing of the map, the cold front had advanced to this 
location. At 0400, he reported flying in clear weather 
showing the clearing between two fronts. At '0500, 
he reported flying above strato-cumulus and under 
cumulus, which condition persisted for his 0600 
report as well. The deduction is that he was then 
crossing the warm front zone. 

The weather map, Figure 49, shows that the low 
over the continent has practically dissipated, as had 
the one over the eastern Atlantic, while the low now 
over the central ocean area has deepened, occluded, 
and its center has moved slowly north-eastward. The 

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Fig. 50. Weather map, February 14, 1940 

















































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Fig. 51. Weather map, February 15, 1940 
















































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Fig. 52. Weather map, February 16, 1940 




























































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Fig. 53. Weather map, February 17, 1940 












































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Fig. 54. Weathet map, February 18, 1940 











































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Fig. 55. Weather map, February 19, 1940 





























































-- — — 


60 


Fig. 56. Weather map, February 20, 1940 





















































high pressure over North America is becoming more 
intense and is spreading over the entire continent. 
The polar front lies in an east-west direction with 
several waves developing. These waves which appear 
along the fronts within the United States frequently 
become severe storms at sea and command the atten¬ 
tion of the aerologist and the respect of flyers. 
Experienced ocean flyers have found it advisable, 
wherever possible, to avoid flying directly through 
the centers of these secondary formations by alter¬ 
ing course sufficiently to skirt the edge of the bad 
weather. 

On the weather map, Figure 50, the wave devel¬ 
opment that appeared on Figure 49 in the middle- 
west has deepened and, reaching the coast, has be¬ 
come an intense storm area. The low in the Atlan¬ 
tic has remained almost stationary and has become 
somewhat less intense, but the occlusion process 
has continued and the occluded front has moved 
farther east toward the European coast. Activity 
is starting in the Mediterranean. The low, that on 
the previous day had approached the coast of Por¬ 
tugal, now brings precipitation to southern Iberia. 

On the weather map, Figure 51, the theory that 
secondaries frequently become more intense than 
the parent storm is well demonstrated because the 
small wave that developed two days previously in 
Tennessee now dominates the weather of the west¬ 
ern North Atlantic. The original low over the Atlan¬ 
tic has moved northeastward, forcing northward the 
high that was over Iceland. The subtropical high has 
also been displaced eastward. The polar high over 
North America continues to dominate the weather 
over that continent. 

On the weather map, Figure 52, there is a con¬ 
tinuation of the movements as described in the 
foregoing paragraph. The original low over the 
western Atlantic (Figure 48) is now occluding itself 
out to the southeast of Iceland, the wave develop¬ 
ment in the middle Atlantic of two days previous 
(Figure 50) is now occluded in the eastern Atlantic, 
and the low originating in the southern states now 
dominates the central Atlantic. 

Weather maps from February 17th to February 
20th (Figures 53, 54, 55 and 56) show the growth of 
the low over the Atlantic and its dominance over all 
the weather from Europe westward over the Atlantic 
Ocean and the continent of North America. Each 
succeeding wave development has pushed itself 
through its life history and occluded itself out in 
the expansive low pressure area. In these final maps 
of the series, this low has set up a large circulation 


which will allow the continental high to swing 
southward, and there exists a large movement of 
air from the tropics to the Arctic over the east¬ 
ern Atlantic, and from the Arctic to the tropics 
over the North American continent and the western 
Atlantic Ocean. It is frequently the case that waves 
will develop along this cold front and the storms 
thus formed will develop and move across Europe 
into Asia. 

As the big low continues to dissipate itself and fill 
up, the cycle of development can then be started 
all over again. It must be understood that no two 
cases are identical and considerable time might elapse 
before even a similar series of cyclonic disturbances 
would occur. However, some such circulation will 
take place in order to complete the transportation 
of the air masses in the world-wide atmospheric cir¬ 
culation pattern. 

The last three maps of this series show a new 
polar front, with cyclonic development, forming 
and intensifying along the Atlantic coast and 
advancing seaward. 

At 2123, nine and one-third hours after map 
Figure 56 was drawn, a plane took off from Ber¬ 
muda for the Azores. The course and cross-section 
of this flight is shown. This plane took off in the 
warm sector of a low pressure area centered off the 
coast of the United States at Latitude 40 °N. The 
pilot pulled up and was flying at 8000 to 9000 feet 
between layers of strato-cumulus and altostratus or 
cirro-stratus clouds. At 0200, he reported he was 
flying in moderate steady rain and light snow above 
stratus clouds and under cumulo-nimbus. This 
would indicate that he was feeling the effects of the 
warm front shown on the weather map. At 0300, 
he had passed through this condition and was again 
over strato-cumulus and under alto-stratus. At 0800, 
he was over cumulus, and, as he was making his 
descent at 0900, he flew through patches of cumulus 
and strato-cumulus and collected some ice. He 
reported freezing drizzle. These cumuliform clouds 
were, no doubt, of air mass origin since no frontal 
activity was evidenced in the area. The plane landed 
at Horta at 0943. 

Weather During Individual Flights 

Figures 57-64 show the weather maps and cross- 
sections of weather in various parts of the Atlantic 
Ocean as experienced on flights made from Sep¬ 
tember 13, 1941, to October 17, 1941. 

Washington to Argentia. The map for September 
13, 1941 (Figure 57) was drawn at 1200 G.C.T. 
The dominating feature of the North Atlantic 

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Fig. 57. Weather map, September 13, 1941 




















































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Fig. 58. Weather map, September 14, 1941 































































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Fig. 59. Weather map, September 28, 1941 






















































according to this map is the cyclonic area (996 mb.) 
centered over the west coast of Greenland. A long 
cold front extends from this center southward to the 
Caribbean, with two waves and the accompanying 
warm fronts developed south of Latitude 40 °N. 
Behind this principal cold front are two succeeding 
secondary cold fronts, one having passed off the 
coast of North America and the other swinging 
down from the Arctic area over Labrador. 

The departure time for the trip from Washington, 
D. C., for Argentia, Newfoundland, was 1222 
G.C.T., 22 minutes after the drawing of the map. 
This flight was made “contact” over the entire area 
at about 2200 feet. 

What was the weather as encountered on this 
flight? 

What effect did wind have upon this flight? 

If you were to make a flight from Washington 
to Bermuda on this day with a departure time of 
1222 G.C.T., what would be your flight plan? 

What would be the flight plan from Washington 
to the Azores? 

Argentia to Stranraer. On September 14, 1941, 
the eastbound trans-Atlantic flight was continued 
from Argentia to Stranraer, Scotland. This flight 
merits your careful study. The weather map 
(Figure 58) was drawn at 2400 G.C.T., after the 
flight was started at 2120 G.C.T. 

The cyclonic area centered over Greenland con¬ 
tinues to dominate the weather over the North 
Atlantic. Two cold fronts and a warm front are 
indicated. Two hundred miles out, the pilot encoun¬ 
tered first evidences of the secondary cold front 
in the form of cumulus clouds. He climbed to an 
altitude of better than 10,000 feet. This, however, 
was insufficient to top the turbulent cloud forms at 
the front, so to cross the front he climbed to 13,000 
feet, flying on instruments while in the alto-cumulus. 
While on instruments he encountered light ice and 
moderate turbulence. After passing over the front, 
the pilot let down through the alto-cumulus and con¬ 
tinued over broken cumulus and strato-cumulus at 
an altitude of about 10,000 feet. About 850 miles out, 
a solid layer of strato-cumulus was encountered with 
tops at about 4000 feet. Other than this low-flying 
blanket of strato-cumulus and scattered alto-cumulus 
at 10,000 feet, no other formations are noted in 
connection with the principal cold front. When 
about 1675 miles out, the pilot attempted to come 
down under the stratiform clouds but was still on 
instruments when only about 100 feet off the water, 
so he climbed on top again. When only a few miles 


from his destination, he again came down through 
the overcast breaking out at about 1500 feet. In 
order to follow this flight and fit the cross-section 
of the flight, to the weather map, it is necessary to 
visualize the movement of the weather while the 
airplane was in flight. Note that the warm front 
shown on the map at 20°W. Longitude had moved 
to within 200 miles of Stranraer by the time the 
flight reached its destination. Also, in making an 
over-the-top flight of such length over water where 
there are no land marks, the importance of wind 
becomes obvious. 

Stranraer to Gander Lake. You had better fasten 
your safety belts securely if you are going to “live 
the part” of the pilot as we follow the return trip 
across the North Atlantic from Stranraer, Scotland, 
to Gander Lake, Newfoundland. The departure was 
made at 1735 G.C.T., September 28, 1941, and did 
not have the advantage of the weather map which 
was drawn at 1800 G.C.T. (Figure 59). The fore¬ 
cast supplied the pilot did not indicate all that was 
ahead of him. 

At the take-off the weather was fair, an occluded 
front had just passed leaving scattered alto-cumulus 
clouds at about 5000 feet; so the pilot climbed to 
3000 feet and leveled off. About 75 miles out, he 
ran into strato-cumulus, and, flying instruments, he 
reduced his altitude to 2000 feet in search of more 
favorable weather. 

About 220 miles out, flying at 2000 feet, he ran 
into clear weather, but with a 30 knot wind from 
260°. Sighting towering cumuli ahead, the pilot 
attempted to top them, but at 6200 feet he could 
see an overcast of alto-cumulus through breaks. 
Identifying this as a front that was not forecasted, 
he dropped down to 2800 feet encountering showers 
on the way down. 

For the next 350 miles he flew through cumulus 
and strato-cumulus clouds encountering violent tur¬ 
bulence, rain, and a headwind of about 50 knots. 

As he passed through the front, he encountered 
sleet, and again attempted to climb out on top. He 
picked up ice at about 5000 feet, but came out on 
top of the clouds at 9000 feet and leveled off at 
10,000 feet. Here a headwind of 70 knots from 
250° so impeded progress that he again dropped 
down to 3800 feet. Soon after, he crossed the sec¬ 
ondary cold front into modifying cP air, with scat¬ 
tered strato-cumulus and decreasing wind velocity. 
The last 700 miles were uneventful, flying over 
scattered strato-cumuli. 


65 



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Fig. 60. Weather map, October 8, 1941 















































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Fig. 63. Weather map, October 16, 1941 




























































70 


Fig 64. Weather map, October 17, 1941 






























































Norfolk to Bermuda. On the weather map for 
October 8, 1941 (Figure 60), is included a flight 
from Norfolk to Bermuda. The weather map was 
drawn at 1200 G.C.T. and departure from Norfolk 
was made at 1312 G.C.T. 

This flight was made contact by flying at about 
6200 feet over scattered cumulus for about 250 
miles, then coming down to 1000 feet to fly under 
broken cumulus clouds with showers. 

What causes the cumuliform clouds between 
Norfolk and Bermuda? 

What type of weather would you encounter, 
according to this map, between Bermuda and the 
Azores? 

Would there be any flying weather hazards 
between Norfolk and Argentia, Newfoundland? If 
so, what are they? 

What weather exists on the route from Argentia 
to Stranraer, Scotland? 

What is the dominant weather feature in south¬ 
eastern United States? 

Bermuda to Horta. On October 9-10, 1941, a 
flight was made between Bermuda and the Azores. 
The departure from Bermuda was made at 2208 
G.C.T., October 9th. The weather map (Figure 61) 
was drawn at 2400 G.C.T., 1 hour and 52 minutes 
after the take-off. 

This flight was made at an altitude of 10,000 to 
12,000 feet, thereby flying above the scattered 
thunderstorms which are associated with the frontal 
activity dominating the weather for the first part 
of the trip. It is interesting to note that the latter 
part of the flight was made in very good weather 
but, had the flight been made a little farther south, 
trouble would probably have been encountered in 
the wave formation southwest of the Azores. Vet¬ 
eran trans-Atlantic pilots have learned tc respect 
these waves and draw their flight plan to circum¬ 
navigate them. 

Draw a cross-section of the weather on the 
Argentia-Stranraer route according to this map. 

Horta to Stranraer. At 0800 G.C.T., October 12, 
1941, the trans-Atlantic plane left Horta, Azores, 
for Stranraer, Scotland. The weather map (Figure 
62) was drawn at 1200 G.C.T., October 12th. 

The first 950 miles were made at 8200 feet over 
scattered strato-cumulus clouds. At the cold front, 
descent was made through cumulo-nimbus to 6000 
feet, where it was possible to fly over broken strato- 
cumulus and under alto-cumulus for about 100 miles. 
Approaching the warm front, further descent was 


made to 2500 feet to fly under the strato-cumulus 
and over the 2000 foot layer of fog which had 
developed under the warm front. About 200 miles 
from Stranraer, the fog bank cleared and the trip 
was completed under an overcast of alto-stratus and 
scattered to broken strato-cumulus at 1500 feet, 
which permitted contact flying at about 1200 feet 
for the last 150 miles of the trip. 

Veteran trans-Atlantic pilots have discovered that, 
in approaching a front from the west, and flying 
over the overcast, if they wait until they can see 
the towering cumulus at the front before starting 
their descent, they will probably come down in the 
most turbulent part of the formation, whereas, if the 
front is anticipated and they start their descent 
before it can actually be seen, they will encounter 
better flying. 

Looking back over the weather maps for this 
flight, what advantages or disadvantages did the 
route via Bermuda and the Azores have as com¬ 
pared to the route via Argentia, Newfoundland? 

Stranraer to Horta. On October 16, 1941, at 
0637 G.C.T., the return flight to the United States 
from Stranraer via the Azores was started. The 
weather map (Figure 63) was drawn at 0600 G.C.T. 

The dominant feature of the trip from Stranraer 
to the Azores was the cold front and its accom¬ 
panying cumulo-nimbus cloud, which had to be 
crossed. Therefore, this flight was made contact at 
altitude under 2000 feet in order to stay under the 
cloud bases which hung at about 1800 feet. Showers 
and some turbulence were experienced in the area 
under the influence of the front (between 400 miles 
and 900 miles from Stranraer). The remainder of the 
flight to the Azores encountered only scattered 
cumulus and some cirrus clouds. Winds on this trip 
were generally light except in the area ahead of the 
front. 

What would have been the weather between 
Stranraer and Argentia according to this map? What 
would have been the winds? Why? 

Horta to New York. On October 17, 1941, at 
1730 G.C.T., take-off from the Azores direct for 
New York was made. The weather map (Figure 64) 
was drawn at 1800 G.C.T. 

A weak warm front accompanied by only scat¬ 
tered cumulus and strato-cumulus clouds at 2500 
feet was encountered about 400 miles west of 
the Azores. At the cold front, 700 miles out of 
Horta, moderate turbulence and heavy precipi¬ 
tation was experienced in the cumulo-nimbus clouds 

71 


which were flown through at an altitude of approxi¬ 
mately 2500 feet. Prior to the crossing of the front 
the winds were 25 to 35 knots from 185° to 240°, 
but after crossing the front they were 30 knots from 


340°, diminishing and veering to the west and then 
southwest as the coast of North America was 
approached. No bad weather was found in cross¬ 
ing the second cold front shown on the weather map. 







72 




C7=^ TROPICAL ATLANTIC -S^D 


North Atlantic airmen transferred to the Caribbean (Figure 65) refer to it as their “rest cure.” The 
weather is reliable in comparison with the more changeable weather of middle latitudes. However, 
tropical weather has its own particular brand of whims. Pilots must cope with varying visibility and 
cloudiness, occasional tropical cyclones, northers, heavy squalls, and torrential rains—all of which may 
seriously affect operations. 


FLYING WEATHER OF THE WEST INDIAN REGION 

The West Indian region is normally covered by 
maritime tropical air with relatively uniform air 
mass weather. The region is relatively front-free, 
except at times during the winter when outbreaks 
of polar air force the polar front southward, and 
occasionally during the summer when the inter- 
tropical front invades the Caribbean. 

Winds 

The trade wind system influences the West Indian 
region throughout the entire year, and normally 
dominates the weather of the area. In the Bahama 
group, easterly winds prevail but with many north¬ 
east winds in winter and southeast winds in summer. 
Over the Greater Antilles (larger islands) and the 
western part of the Caribbean Sea, northeast winds 
prevail in winter and east winds in summer. Over the 
eastern islands, the trades blow with great con¬ 
stancy from points between east-northeast and east- 
southeast. Let’s call this a region of easterly trades 
(Figures 8, 9, 66 and 67). 

Although the prevailing wind system of this 
region as a whole is simple, there are seasonal and 
diurnal changes that influence flying. 

Seasonal Wind Velocities. Over most of the 
West Indies region midsummer winds are strongest, 
with lighter winds in spring and fall. Over the Gulf 
of Mexico and the Bahamas, however, late fall and 
winter winds average strongest, with occasional 
strong gales blowing from a northerly direction 
(Figures 68 and 69). 

Land and Sea Breezes. The winds at coastal 
stations on all but the smallest islands are more or 
less affected by the land and sea breezes common 
to most tropical coasts. At many places these winds 


blow regularly and vigorously almost daily through¬ 
out the year. 

On leeward* coasts of the larger islands the land 
and sea breezes are best developed. Western Haitian 
coast towns, for example, experience sea breezes 
starting as early as 1000 or 1100 and lasting until 
after sundown. On the leeward coasts the sea breezes, 
of course, blow from a direction opposite that of the 
trades, being drawn in by low pressure caused by 
heating of the land surface. At night, as land cools, 
the land breeze sets in. (The surface wind roses for 
Kingston in Figures 66 and 67 show the land breeze 
of early morning.) On leeward coasts of the smaller 
islands the prevailing trade wind merely weakens by 
day and accelerates by night. 

On all windward* coasts the prevailing wind 
becomes strongest during midday and weakest just 
before sunrise. On north and south shores , between 
extreme windward and leeward exposures, the land 
and sea breeze makes itself evident by turning the 
winds more landward by day and more seaward by 
night. On all except leeward coasts, the strongest 
breeze of the day comes at about 1400, and its 
velocity is at least two or three times that of the 
early morning breeze in the lull before sunrise. 
Figures 70 and 71 summarize the land-and-sea-breeze 
effects on windward and leeward coasts of large 
islands. 

The presence of hills and mountains near the shore 
accentuates the land-and-sea-breeze effects by drain¬ 
ing night-cooled air from the land down to the coast 
and by concentrating and accelerating the daytime 
sea breeze in the valleys. The sea breeze may feed 
moist air into the valleys, leading to very heavy 
“stationary” showers on the mountains. 


• “Leeward” and “windward” are used here from the stand¬ 
point of the prevailing trade wind direction. 


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Fig. 65. Locational and physical map of the Tropical Atlantic 


75 
























90° 


80° 


70° 


60° 




3 0‘ 


,*L 


20 °- 



10 ' 


0 ° “ "Tar- 

' 


10 ' 


20 ‘ 


x> 


’ v 


GULF 

MEXICO 


Bermuda 


<s> 


<f SURFACE cnnd{ ALOFT 

For ^ich station: 


Lope/- wind rose 
*SURFACE wi4 
Upper tfind rose 
winds J\LOF) 


^presents 

ds. 

represents 
at 6000 H. 


30 c 


20 ' 


CARIBBEAN SEA 

% Jf- 


Mi i 


■t v Barrantfuftti 


.. A / 


V GALAPAGOS fS 


W- 



10' 


^ONATTO READ A WIND ROSE 


iVSND DIRECTIONS 
t ’re shown by the direction 

> 'ROM which linefreach \ 

y PERCENTAGE OF d%$ERi 
tN WHICH WIND BLOWS 
EACH DIRECTION is shok 
, y length of each tine. 



. WE RAGE VELOCITY OF W/ND 
, ROM EACH DIRECTION o 
s shown by Beaufort Symbols: 

PERCENTAGE OF OBSERVATIONS WITH CALMS 


' s represented by nurhet ’a! in circle 


VAT/ONS 

From 

50 30 io % 

K 444 H 0 

O 40 20 0 % 



10 c 


20 ‘ 


90° 


80° 


70° 


60° 


76 


Fig. 66 



































































90 ° 


80 ° 


70 ° 


60 ° 


30° 


20 



10 


10 


20 




Bermuda 


% 


SUMMER WINDS I 

<T SURFACE and ALOFT 

• 

For ’T&ich station: 

Lower wind rose Represents 
SURFACE Winp/s. 

Upper tfnnd rose 

winds ^ALOFYat 6000Ft. 


yr? \< 

CARIBBEAN SEA 

k ^, - 



0 °—f-.-- 


HOV/^TO READ A WIND ROSE 

s' 

WIND DIRECTZjQNS 


* GALAPAGOS fS 


the 




t -ROM which iinef reach 

i PERCENTAGE OF dSsERVA 
N WHICH WIND BLOWS FROM 
-ACH DIRECTION is she 
i ?y length of each //he: ■ 

AVERA6E VELOCITY OF 
FROM EACH DIRECTION 
, s shown by BeauFort 5 


PERCENTAGE OF OBSERVATIONS MTJLCAim 


/s represented by numeral in circleF? 


in M 




Sp ain — 


NW I NE 

circle: 


T/ONS SW £ S£ 


50 30 10 % 

H » W HO 

O 40 20 0% 

Wind 

o 

>j//nbols: 




BeauFort 

number: 


0 

~T 
2 

3 

4 

5 

6 

7 

8 

9 

10 
11 
12 


Map Velocity 
Symbol in knots 



o 

m —O 

Vtttti Q 
WWW Q 


Leisthari 1 
; 1-3 

U-6 

7-10 
11-16 
17-21 
22-27 
28-33 
34-40 
41-47 
48-55 
56-65 
Above 65 


30 c 


20 


10 


10 


20 ‘ 


90 c 


80° 


70° 


60 c 


Fig. 67 


77 































































78 


Fig. 69 










































Fig. 70 


TRADE WIND 


Fig. 71 


WEAK 
SEA BREEZE 


STROKE 
SEA BREEZE 


The Central American and South American coasts 
have more pronounced land-and-sea-breezes than the 
islands, because of the strong continental daytime 
heating. 

The land-and-sea-breeze phenomenon is purely 
local. Its influence does not extend far from land 
and seldom extends to a height of 3000 feet. The 
trade wind, on the contrary, is a deep current extend¬ 
ing over a vast area. As soon as one proceeds a 
relatively short distance from shore, the trade is 
found to be the predominant wind. 

Winds Aloft. Wind velocities increase up to 
about 4000 feet, reaching greater velocities in mid¬ 
summer than in other seasons. Above 4000 feet 
velocities level off and, in suntmer, the velocity of 
wind aloft over the West Indies actually decreases 
as one ascends from 10,000 feet to about 30,000 feet. 

Aloft above the easterly trade winds, wind direc¬ 
tion is reversed. When flying high over the West 
Indies (or any trade wind region) you’ll find west¬ 
erly anti-trades (generally west-southwest). 

The level of reversal from easterly trade to west¬ 
erly anti-trade varies with the seasons—the trades 

§ extend highest in summer and are most shallow in 
winter. It also varies with latitude—the trade winds 
increase in depth toward the equator. 



At Miami and Key West the easterly trades pre¬ 
vail up to about 5000 feet in winter and up to about 
12,000 feet in summer. At Kingston, Jamaica, and 
San Juan, Puerto Rico, easterly trades prevail up to 
about 15,000 feet in winter and higher in summer. 

The boundary between easterly trades and west¬ 
erly anti-trades shifts up and dawn from day to day 
over a large range of height. This causes a zone aloft 
of extremely variable winds. Notice, for example, 
the variable winds at 6000 feet over Key West in 
winter (Figure 66)—in summer, the zone of variable 
winds of course lies above the 6000 feet level 
(Figure 67). 

Northers 

Disturbances of higher latitudes occasionally affect 
the winds and weather of the West Indian region, 
especially in winter and less frequently in spring and 
fall. 

When a cold wave of winter advances southward 
over the Mississippi Valley, it sometimes sends a 
surge of chill air equatorward over the Gulf of 
Mexico and on to the coasts of Mexico and Central 
America or across the West Indies. This norther 
(or El Norte) may retain its chill far into the tropics. 
Though most pronounced in western portions of the 
region, it has lowered Cuban temperatures to the 
6o’s and may influence the weather as far south and 
east as Trinidad. The norther may break onto the 
Gulf of Mexico, the Florida Straits and the Bahamas 
with gale force and accounts for the distribution 
of winter gales (shown in Figure 68). As the cold 
air moves over warm water its lower layers, becom¬ 
ing unstable, produce cumulus clouds and showery 
weather. 

Along the eastern coast of Mexico and Guatemala, 
and sometimes farther east, a sharp cold front may be 
felt, with high winds (up to 50 knots) and blustery, 
overcast weather. Torrential rains fall where the 
front rises against the mountains. More commonly, 
however, and especially farther south, invading air 
becomes so modified over the sea that it meets the 
trade air with only enough temperature difference 
to produce weak frontal activity. This frontal activ- 

79 














Fig. 72. Weather map, November 13, 1940 



Fig. Ti • Weather map, November 14, 1940 











































Fig. 75. Weather map, November 17, 1940 








































Winter blowing over warm Gulf building thunderstorms 


ity occurs along a low-pressure trough that extends 
SW.-NE. through the Gulf of Mexico of the West 
Indian region. It is commonly the warm, mild end of 
a vigorous polar front of winter that lies off the 
east coast of the United States (Figure n), and, as 
such, it moves eastward through this region. Many 
winter weather maps show a trough of this type, 
adding a touch of the extratropical to the generally 
tropical weather of the West Indies. 

As you learned relative to the polar front of 
middle latitudes, there is a tendency for cold air 
outbreaks from the polar highs to recur during the 
winter at intervals of 5 to 8 days. This produces as 
many as 30 northers per year in the Gulf of Mexico. 
To the east and south, however, the number de¬ 
creases greatly, because most strong northers enter 
the Gulf from Texas and the cold air warms rapidly 
over the water. 

Almost every time a vigorous cold front or a 
well-developed eastward-moving trough passes just 
north of the Caribbean it causes some disturbance as 
far south as Panama and Trinidad. As a norther 
approaches, the area to the south of it receives more 
southerly winds with slightly warmer and more 
humid weather. Over the Gulf of Mexico, and in the 
Florida Straits, the Bahamas, and sometimes south¬ 
ward to Jamaica, the approach of the norther may 
be heralded by a heavy cloud bank on the northern 
or northwestern horizon. Its arrival may bring a 
severe squall. With the passage of the front or 
82 


trough, winds normally set in from northeast, north 
or northwest, bringing Cooler but cloudy, showery 
weather. 

In Puerto Rico the norther may cause a consid¬ 
erable drop in temperature and strong northerly 
winds, and may cause steady (sometimes heavy) 
rains for a day or more, with low overcast sky, 
heavy sea swells, and fresh winds or gales. But true 
northers reach Puerto Rico less than twice a year! 
The cold fronts that appear with true northers far¬ 
ther north move on past Puerto Rico as weak 
troughs. These troughs bring increased cloudiness 
and rain as the only discomforts to airmen. Some¬ 
times, however, without any fall in temperature or 
shift of surface winds, the effects of cold air out¬ 
breaks continue over the Lesser Antilles to Trinidad 
in the form of a belt of augmented winds and wide¬ 
spread cloudiness. 

Weather Maps Showing Norther in the West 
Indies. Figures’72, 73, 74, and 75 show the migra¬ 
tion of a norther which pushed across the Gulf of 
Mexico into the Caribbean (November 13 to 17, 
1940). Examination of these maps will demonstrate 
to you the relationship of the weather characteristics, 
discussed in the foregoing paragraphs, to a polar out¬ 
break (cold front on maps). 

Hurricanes 

About 7 hurricanes (tropical cyclones) ravish the 
West Indian region in an average year, but few years 
fit the average. The annual number of storms may 
vary from one to more than 20. They may strike 
anywhere within the region. 

To a Carib Indian, the word “hurricane” means 
“big wind.” To a Naval pilot, a hurricane warning 
means “that’s no place for me.” 








Fig. 76. Pressure and wind system of a tropical cyclone of the 
Northern Hemisphere 


The dangers associated with hurricanes are: the 
destructive force and violent shifting of the wind, 
torrential rain that reduces visibility, mountainous 
seas, and storm waves or tides. In short, AVOID 
hurricanes like the plague. Hurricane weather is 
NOT flying weather. Planes on the ground should 
be under shelter if possible, or at least most securely 
tied down. The hurricane of the tropical North 
Atlantic is a tropical cyclone similar to the typhoon 
of the China Seas, the baguio of the Philippines, 
the cyclone of the North Indian Ocean, the willie 
willy of the Timor Sea, the cordonazo off the Pacific 
coast of Mexico, and the hurricane of the South 
Pacific. 

Nature. The hurricane is a “big wind” that 
blows in a circle—an enormous whirl, with winds 
blowing counterclockwise around the center and 
spiraling inward (Figure 76). (Around hurricanes 
of the South Pacific, on the contrary, winds blow 
clockwise.) The angle of indraft is about 30°. If the 
observer turns his back to the wind, therefore, the 
storm center will be ahead and to his left (in the 
southern hemisphere, ahead and to his right). 

The diameter of the area covered by the entire 
wind circulation may be anywhere from 50 to 800 
miles. On the outer limits of the storm the winds are 
moderate but characteristically gusty. The force of 
the wind increases in intermittent squalls as the 
center approaches, with strongest winds near the 


center. In severe hurricanes, velocities of 100 knots, 
or even 150 knots, may be maintained for periods of 
several minutes and gusts reach still greater veloci¬ 
ties. A nearly calm “dead eye”, with a diameter of 
5 to 25 miles, lies at the center of the storm. 

Throughout the entire hurricane area the sea 
tosses in turmoil, and great tidal waves sweep out 
ahead of the storm to warn of its approach and to 
do their quota of damage to coastal stations. 

At the outer limits of the storm, showers fall. As 
the center approaches, squall showers increase in 
frequency and intensity. Nearer the center, rain is 
continuous and heavy to excessive, falling from 
dense nimbo-stratus and cumulo-nimbus. Excepting 
the “dead eye”, which commonly remains clear, 
clouds overcast the storm area, and high cirrus 
extends far out around the edges. The cirrus is 
followed by alto-cumulus, and then the dark bank 
of nimbo-stratus, termed the “Bar of the Storm,” 
comes over the horizon. 

Pressure may drop to as low as 28" (948.2 mb.) 
or 27" (914.3 mb.) or even lower, and great updrafts 
occur in the excessive storm belt surrounding the 
“dead eye”. These updrafts build tall cumulo-nimbus, 
and cause winds at high levels to blow outward from 
the storm. 

Figure 77 shows diagrammatically what happens 
when a strong hurricane passes directly over. Don’t 
let it happen to you! 



Fig. 77. This is what happens when a strong tropical cyclone 
passes directly over 


83 



























































HURRICANES OF THE WEST INDIES 


Month 

Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

Year 

Average 

Frequency 





.01 

.40 

.45 

1.50 

2.15 

1.62 

0.32 

.05 

6.50 


Origin. There are two principal places of origin 
of West Indian hurricanes: (i) the western Carib¬ 
bean Sea and (2) the tropical Atlantic just south of 
the Cape Verde Islands. Occasionally, however, hur¬ 
ricanes may originate in other sections of the warm 
Atlantic about io° and 12 0 north of the equator. 

Wherever warm moist air over a broad area causes 
many convectional showers, the pressure falls and 
a gradual inflow of air takes place. If this occurs at 
a sufficient distance from the equator, the winds 
deflect and a whirl forms. Hurricanes can’t start near 
the equator. 

Season of Occurrence. Doldrum belt conditions, 
ideal for the creation of hurricanes, lie farthest north 
(farthest from the equator) in the Atlantic during 
late summer and fall. The above table shows the 
average number of West Indies hurricanes, by 
months. 

Nearly all of the hurricanes that originate in the 
region south of the Cape Verde Islands develop in 



the months of August and September, when the 
doldrum belt lies farthest north over that ocean area. 

The Pacific doldrum belt extends into the western 
Caribbean Sea in June and again in October. Octo¬ 
ber is the month of greatest frequency for hurricanes 
originating there, with some forming in late Septem¬ 
ber and early November, and with a secondary 
maximum in June. 

Tracks. Nearly all of the hurricanes which origi¬ 
nate in the Cape Verde Islands region first move in a 
westerly and northwesterly direction over the Atlan¬ 
tic and later recurve to move in a northerly or 
northeasterly direction. A few from this Cape Verde 
area, however, continue westward to the coasts of 
the United States or Mexico and, without recurving, 
move into the interior to dissipate. Examples of 
tracks with and without recurve are shown in Fig¬ 
ure 78. Remember that these hurricanes from the 
Cape Verde area, with the types of hurricane tracks 
shown in Figure 78, are most numerous in August 
and September. 



Fig. 78. Typical tracks of hurricanes originating Fig. 79. Typical tracks of hurricanes originating 

in the Cape Verde region in the western Caribbean 


84 






































Those few hurricanes which form in the western 
Caribbean Sea in June and July generally pass north¬ 
ward or northwestward into the Gulf of Mexico 
and reach the coasts of the United States or Mexico 
(Figure 79). In September and November, however, 
hurricanes from the Caribbean tend to start north¬ 
ward, then recurve and move northeastward over 
southern Florida or the Greater Antilles (Figure 79). 

Many courses followed by hurricanes have been 
quite irregular—there are no fixed or average paths. 
Figure 80 shows tracks of hurricanes with whims. 
Never trust them. 

Quite commonly one of these tropical terrors, 
after recurving, joins forces with one of the much 
milder cyclonic depressions along the polar front. 
This union starts the temperate disturbance on a 
brief spree but rapidly reduces the virility of the 
tropical storm. 

Speed. The progressive forward movement of a 
hurricane, while in the tropics, averages about 10 
or 15 knots. During the recurve, many tropical 
storms move very slowly, and some remain almost 
stationary for a day or more. After the recurve, the 
storm generally increases its forward speed, some¬ 
times moving at a rate of 40 knots in higher latitudes. 

In peacetime, with more observations available, 
it is generally possible to forecast the path of a 




Fig. 81 

hurricane and AVOID it. However, with the 
paucity of reports available during a war, the pilot 
should know (a) as much as possible about nature’s 
warning signs so that he won’t unsuspectingly blun¬ 
der into one, and (b) which suburb, once in the 
neighborhood, is the least uncomfortable. 

Signs of Approach. Tropical weather can be 
quite monotonous from day to day. (Transport 
pilots can stand quite a lot of this type of monot¬ 
ony.) The approach of a hurricane is sometimes 
suspected when still a long distance away, because 
of relatively slight changes from prevailing condi¬ 
tions for the season. Signs of approach include: 

A sea swell. Long, unbroken waves appear on the 
sea with the time interval between crests consider¬ 
ably longer than for ordinary waves. This sea swell 
may precede destructive winds by several hours. As 
the storm approaches, the sea becomes much rougher 
and the tide rises above its normal height. 

Cirrus and cirro-stratus clouds. These may extend 
hundreds of miles ahead of the approaching hurri- 

85 


Fig. 80. Tracks of hurricanes with whims 





















86 


Fig. 82. Weather map, October 16, 1943 (1230 G.C.T.) 
















Fig. 83. Weather map, October 16, 1943 (0030 G.C.T.) 


87 































88 


Fig. 84. Weather map, October 17, 1943 (1230 G.C.T.) 




























cane (but they may also occur when no hurricane 
approaches). Streaks of these high feathery clouds 
often appear to converge at a point on the horizon. 
That point of convergence may indicate the direc¬ 
tion in which the storm center lies. 

Brilliant red sky at sunset and sunrise. Caused 
by coloring of the cirrus, this is one of the well- 
known signs of an approaching hurricane. 

Wind increasing in velocity. A safe guide, but 
listen before this warning shouts! Wind direction is 
the safest guide as to the direction of the storm 
center. 

The “Dangerous Semicircle.” The right half of 
a Northern Hemisphere tropical cyclone (as viewed 
when looking forward along the cyclone’s path) is 
known as the “dangerous semicircle”. (In the 
Southern Hemisphere, the left half of a tropical 
cyclone is the “dangerous semicircle”.) 

There are two reasons: (i) A ship or an aircraft 
caught in the right half of a tropical cyclone of the 
Northern Hemisphere may be blown toward the 
path over which the storm vortex will pass (Figure 
81); or the recurving of the storm may direct the 
vortex toward the ship or aircraft. (2) Wind veloci¬ 
ties have been found to be generally higher in the 
right half of the storm.* 

Which Direction to Fly. The so-called signs of 
approach of a hurricane rarely indicate that you are 
near an edge of the storm. Wind direction will reveal 
what direction the storm center lies from you. Apply 
the Buys Ballot Law. In the Northern Hemisphere, 
if you turn your back to the wind the low-pressure 
center will be to your left. Notice how this applies 
relative to Figure 81. 

Knowing (1) what direction the storm center 
lies front you and (2) the probable course it will 
follow, you have a basis for judging which way 
to fly. 

Avoid the “dangerous semicircle”! 

Avoid the entire storm, if at all possible. 

Weather Maps Showing a Hurricane. The 
accompanying series of weather maps (Figures 82, 
83 and 84) show the history of an individual hurri¬ 
cane. The most outstanding feature of the storm is 
that it did not follow one of the usual paths (Figure 
79), but took a course almost due north at some 
distance out to sea. 

* In this half, the speed of translation is added to the normal 
wind velocity. 


Having originated in the Caribbean, it moved 
slowly across the West Indies for approximately 12 
hours and then increased its speed. Figure 82 shows 
its path and its location after 48 hours of its recorded 
history. Figure 83, drawn 12 hours later, shows the 
storm has intensified from 993 mb. to 990 mb., and 
maintained its accelerated speed and northward 
course. Figure 84, drawn 12 hours after Figure 83, 
shows the tropical cyclone joining the extratropical 
cyclone. It will soon lose its identity as a hurricane. 

This series of maps further demonstrates how 
ocean weather maps have to be drawn in wartime. 
Ship’s radios are silent. The storm’s position and 
intensity must be determined from conditions at 
the permanent stations. This storm passed close 
enough to Bermuda to permit fairly accurate com¬ 
putation of its intensity. Note the changes in wind, 
sky, and barometric tendency at Burmuda on the 
three maps. 

Thunderstorms 

Fewer thunderstorms occur in the West Indies 
region than the warm temperatures (Figures 85 and 
86) might lead us to expect. Storms with lightning 
and thunder strike chiefly in the warmest months 
(May—October), and most of them occur near 
coasts and over land areas. Windward mountainous 
districts of mountainous islands and coasts have the 
greatest number of thunderstorms—with probably 30 
to 150 thunderstorm days per year. Over low flat 
islands thunderstorms may develop on hot moist days 
when only gentle breezes blow (Figure 87). The lee¬ 
ward portions of flat islands have more storms than 
windward portions. Storms may even develop off flat 
leeward coasts, and be blown to sea by the surface 
wind. The thunderstorm frequency over land shows 
a pronounced peak at about 1400, with very few 
storms at night. 

During those occasional summer periods when the 
doldrum belt invades the Caribbean, thunderstorms 
become numerous over the affected ocean area— 
which means chiefly the western Caribbean. Else¬ 
where over the seas of the West Indian region 
thunderstorms sometimes develop—but on probably 
not more than five days (or nights) a year. Over 
these tropical seas most thunderstorms come at night, 
when darkness makes them most difficult to navigate. 

Most inhabitants of the West Indies have never 
seen hail, for it seldom reaches the surface; but a 
pilot who flies through a thunderstorm may find it, 
and may find turbulence as severe as in thunder¬ 
storms of temperate climates. A special hazard exists 
near the storms along mountainous coasts. Cold air 
from under mountain thunderstorms often rushes 


89 








Fig- 85. 



Fig. 86. 


90 
































































































downslope and seaward with sufficient force to 
prevent aircraft operation. 

Except when an intertropical front develops in 
the western Caribbean, or when a norther drives a 
squall line onto the Gulf of Mexico or across the 
Florida Strait, the thunderstorms of the West Indian 
region are of the air mass variety—easily flown 
around. 

Visibility 

The trade air—dominant over the West Indies— 
normally has visibility of more than 20 miles. Vision 
in this region affords no such problems as in the fog- 
draped Newfoundland and Aleutian areas; but, on 
occasion, obstructions to vision do occur in this 
region. Fog, salt haze, dust haze, and falling rain 
cause most of the trouble. 

Fog is practically unknown over the seas of the 
West Indian region. Figure 88 shows much winter 
fog along the Gulf Coast of the United States and 
extending a short distance out over the Gulf, but it 
shows fogless seas farther south. In interior valleys, 
however, night radiation fog sometimes occurs; 
and along the many mountain ranges upslope fog 
or clouds may obscure vision at any time. 



Fig. 87. Thunderstorms in the tropical Atlantic as seen 
from a Naval Air Transport plane (U.S. Navy photo) 


Salt haze can become as thick as light fog. It de¬ 
velops after a period of rough seas which throw salt 



JAMAiCA HAITI 


Percentage of 
A// Observations 

with FOG- 

MODERATE to DENSE 

(Limiting Visibility to % Mile or Less) 

DEC - JAN • FEB. 




Fig. 88 


91 




















































particles into the air. When the relative humidity is 
as high as 75%, and it usually is, the salt particles 
liquefy. Sometimes this salt haze will act as a mirror 
to a pilot flying above the haze. It is then impossible 
for the pilot to see his carrier even though he may 
be only a few miles away from it. 

Dust haze sometimes lowers visibility off all desert 
or dry-land coasts. It occurs off the northern coast 
of South America in the drier (winter) season, but 
seldom lowers visibility to less than ten miles. 

Falling rain causes a common visibility hazard of 
this region. Most of the rain comes in scattered in¬ 
stability showers from cumulus clouds that do not 
attain thunderhead proportions. These scattered 
showers produce the most common changes in trop¬ 
ical weather. Heavy orographic rains may lower the 
visibility of mountain areas, and low visibility due 
to excessive downpour constitutes a principal hazard 
when flying under thunderstorms at sea. In addition 
to these forms of rain, a fine-dropped drizzle-like 
precipitation frequently falls at sea from low strato- 
cumulus clouds produced by turbulence in the sur¬ 
face layers. 

The maximum rainfall frequency over tropical 
seas occurs at night. 

FLYING WEATHER 

BETWEEN TROPICAL SOUTH AMERICA 
AND TROPICAL AFRICA 

Because the equatorial Atlantic narrows to only 
1640 miles between South America and Africa, it 
has become a significant area for both air transport 
and air patrol. Peacetime aviation, blazing the sky 
routes across this “Strait of the Atlantic”, discovered 
equatorial weather problems which now confront 
Navy pilots. 

Weather in the Intertropical Front* 

Lying along the zone of convergence between the 
trades from the Northern Hemisphere and the trades 
from the Southern Hemisphere (Figures n and 12), 
the intertropical front causes most of the flying 
hazards of the equatorial Atlantic. 

The intertropical front always extends across the 
Atlantic, but it is not always an active front. Some¬ 
times, and in some places, it is a single front between 
the two trade wind systems. The frontal weather is 
then confined to a narrow belt 25 to 50 miles wide. 
At other times and places it becomes a wide belt of 
variable winds, calms, and strong convection, on 

* Sometimes called the equatorial front. 


either side of which the trade winds blow. In the 
latter form a frontal belt usually about 100 to 200 
miles wide—it is commonly called the doldrum belt. 
When a strong surge of the trades from one of the 
hemispheres (or an advance of the trades from both 
hemispheres) causes the edges of the doldrum belt 
to converge, active weather may develop on both 
sides of it, giving the effect of two fronts. If the 
push continues and the two fronts meet, the dol¬ 
drums disappear and a single front develops (see 
Figure 8.) The transition from single front to dol¬ 
drum conditions is caused by cessation of the trade 
winds on one or both sides of the front. This transi¬ 
tion may be so rapid, on occasion, that an area, which 
has had definite wind streams and a single front on 
one day, may, in 24 hours, show a doldrum condition 
hundreds of miles wide. Likewise, transition from 
doldrum to single front may be rapid.** 

Varying between a doldrum belt, a combination 
of fronts, and a single front, the intertropical front 
extends across the Atlantic. You will generally strike 
it when flying along the South American coast or 
when flying from Natal to Africa. In the daytime 
you will cross it with less difficulty when flying over 
the sea, for over water it is narrower and more 
clearly defined than over land. Very heavy rainfall, 
severe turbulence, and hail are hazards of the front; 
and generally you will experience least difficulty if 
you cross the front at low altitude.*** 

Seasonal Migration. Figures 89, 90, 91, and 92 
show average locations of the intertropical front of 
the Atlantic in February, May, August and Novem¬ 
ber. In response to the seasonal migration of the 
direct rays of the sun, the intertropical front migrates 
with the heat equator. You will notice that, over the 
Atlantic during the northern summer, the front lies 
well to the north of the geographical equator. Dur- 

** These transitions and variations between doldrum and single 
front conditions will be more fully treated in Chapter 6, on the 
tropical Pacific. 

*** One ferry pilot who had been crossing this belt regularly 
for a long time had this brief summary to make about the 
intertropical front: “Sometimes it is there and sometimes it isn’t. 
It moves north and south with the seasons. Sometimes it isn’t 
very bad and at other times it can be pretty tough. Once a 
friend of mine was going out and the meteorologist told him it 
was pretty bad, but he could go out and have a look and, if he 
didn’t like it, he could come back. He ran into a terrific down¬ 
pour and had a hard time getting back—just on account of the 
heavy rain. Even after he landed the water ran out of the fuse¬ 
lage and wings like it was coming out of a fire hose. But, as I 
say, sometimes it’s bad and sometimes it isn’t. You have to watch 
what you are doing.” 


92 




AVERAGE 

POSITION 

OF 

INTERTROPICAL 

FRONT 


FEBRUARY 


MAY 


AUGUST 


NOVEMBER 



MEAN POSITION DAY BY DAY SUBSIDIARY 

OF FRONT MOVEMENT OF FRONT SQUALL ZONE 
















































ing the southern summer, the front remains slightly 
north of the equator except near continental coasts. 
(Over the continents, during the southern summer, 
the intertropical front migrates well to the south.) 

Doldrum Weather. Where and when doldrums 
prevail, intermittent rain squalls commonly charac¬ 
terize the weather. Upward-moving convectional 
currents cause calms over the warm sea surface. Ver¬ 
tical rise of warm moist air leads to growth of tower¬ 
ing cumulus and cumulo-nimbus clouds. Indrafts 
toward the centers of convection produce variable 
winds, and the convectional storms produce strong 
squalls. At an edge of the doldrum area, trade winds 
occasionally undercut the more stagnant doldrum 
air (when an edge of the doldrum area is aggresively 
closing in). This produces line squalls. On other 
occasions, at an edge of the doldrum belt, trade air 
may overrun the more stagnant air (warm front 
fashion). This may produce thunderstorms and 
generally causes widespread alto-stratus clouds that 
yield rain or drizzle. Ceilings under doldrum belt 
clouds often average about 1000 feet and may lower 
very close to sea level. Cloud tops commonly extend 
up to 15,000 to 25,000 or even 40,000 feet. Visibility 
becomes poor in the rain squalls and drizzle, and ex¬ 
cessive turbulence accompanies the vertical cloud 
development. 

Where the doldrum belt extends over land areas, 
squalls are most numerous and virile during after¬ 
noon and evening hours. Over oceans, however, most 
severe squalls occur at night. 

Not all doldrum areas have the difficult flying 
weather described above. When the trades, on both 
sides of a section of the doldrum belt, overrun the 
doldrum air and meet to blow as a steady stream 
above it, fliers meet few hazards. During the more 
stable hours of the day, rain squalls and thunder¬ 
storms may be widely scattered. In all but the most 
severe doldrum conditions, storms can be dodged 
and weather trouble avoided by “heads up” flying. 

Best flight altitude through a doldrum area with 
many tall cumulus formations is generally around 
800 feet. 

The Single Front. When doldrum belt edges 
close in, making a single and simple front between 
the trade wind system of the Northern Hemisphere 
and that of the Southern Hemisphere, flying condi¬ 
tions through this front depend upon the contrasts 
between the two meeting air masses—contrasts in 
wind direction and in air temperature. Air tempera¬ 
ture difference between NE. trades and SE. trades 
seldom amounts to more than 2 °F.—usually less. 

94 


During late spring and late fall, air masses from the 
two hemispheres have least temperature contrasts. 
With winter in one of the hemispheres (Jan.-Feb. 
and Aug.-Sept.), however, outbreaks of polar air 
join the trades. Though the polar air becomes greatly 
modified before reaching low latitude, these polar 
outbreaks stimulate intertropical front activity. 

Wind directions in the two trade systems may be 
close to parallel near South America. Near the Afri¬ 
can coast, however, they commonly have a greater 
contrast, and this contrast is most marked in August- 
September when the intertropical front lies farthest 
northward from the equator (Figures 11 and 12). 
(Recalling the factors that influence wind direction, 
can you explain why the wind system near the 
Guinea Coast has strongly opposed wind direction?) 

Owing to wind direction and temperature con¬ 
trasts, then, August-September is the season of 
greatest activity along the intertropical front. 

A simple side-by-side flow of the two trades, with 
only slight mixing of the two similar air masses, will 
produce a certain amount of cloud and showers but 
no intense activity. With a 2°F. or 3°F. temperature 
contrast, however, and a conflict in wind direction, 
the intertropical front may develop a line of tower¬ 
ing cumulo-nimbus clouds and a severe squall. Over 
ocean, pilots commonly fly under it. Over land, low 
visibility makes this hazardous. Wherever possible a 
well-developed intertropical front should be circum¬ 
navigated. 

Cyclonic Waves on the Intertropical Front. 
Excepting hurricanes and typhoons and similar mem¬ 
bers of the tropical cyclone family, and the easily- 
dodged thunderstorms, the most serious flying hazard 
of the tropics are occasional cyclonic waves on the 
intertropical front. Like the extratropical cyclones 
of our home latitudes, these waves develop low pres¬ 
sure centers and advance along the front. Unlike the 
extratropical cyclones, which move from west to 
east, the waves of the Atlantic intertropical front 
move from east to west—carried by the currents of 
the trade winds. They originate in the Gulf of 
Guinea or off the West African coast and are most 
frequent and intense in the August-September season. 

Conditions conducive to formation of these waves 
include: (1) sufficient distance from the equator to 
produce adequate wind deflection (no waves form 
along the equator), (2) conflicting general wind 
directions in the meeting air masses, (3) tempera¬ 
ture contrasts in the meeting air masses, (4) much 
moisture in at least one of the air masses. The August- 
September season off the African coast fulfills these 
conditions well. 


Birthplace of Hurricanes. As you have learned, 
many of the hurricanes that disturb the West Indian 
region originate south of the Cape Verde Islands, 
and they come from this area chiefly in the months 
of August and September. They are demonic prod¬ 
ucts of the intertropical front. 

Tornadoes of the Gulf of Guinea. Though 
they carry the same name as the “twister” of the 
American plains (possibly the name was “imported” 
by former American slaves), these are actually large 
and intense thunderstorms. They frequent the Gulf 
of Guinea and the Guinea Coast, with maxima of 
about three a month in May-June and again in 
October-November. They move from east to west 
as short line squalls rarely exceeding 40 miles from 
north to south. Like most storms over oceans and 
coasts they reach greatest intensity at night and 
carry all of the characteristics and hazards of severe 
thunderstorms. 

Trade Wind Weather 

On either side of the narrow belt of bad flying 
weather of the intertropical front—in the trade winds 
to the north and to the south—pilots generally find 
good flying conditions. As the mT air of the trades 
advances toward the intertropical front, its lower 
layers warm and develop cumulus clouds. The mild 
turbulence associated with these clouds, and the light 
showers they scatter over the ocean, cause pilots 
little trouble. Only on mountainous windward slopes 
of islands and continents do low ceilings develop and 
heavy rains fall in the trades. 


The seasonal migration of the intertropical front 
must be considered relative to all flying between 
South America and Africa. In the Cape Verde Islands 
(15 0 to i7°N. Latitude) for example, the northeast 
trade winds blow during 300 to 340 days in a year. 
During the remaining time, however, within the 
months of August and September, winds become 
more variable and sometimes blow from the south¬ 
west. The Cape Verde Islands, feeling the influence 
of the intertropical front, then suffer their rainy and 
stormy season. 

To the south of the equator, on the contrary, the 
Atlantic intertropical front seldom extends—except 
along the South American and African coasts (Fig¬ 
ures 89, 90, 91, and 92). Ascension Island, for ex¬ 
ample (8°S. Latitude), has constant trade winds 
throughout the year. Pilots always find good flying 
weather over this area. 

Haze and the Harmattan. The ocean off the 
west coast of Africa, from the Guinea Coast to north 
of Cape Verde, is one of the haziest regions on earth 
—especially in the dry season. Blame the harmattan. 

A dust storm that moves with an east wind from 
the interior of Africa, the harmattan carries dust 
far over the Atlantic. Within the storm area, visi¬ 
bility becomes seriously lowered for two to four 
hours—often to less than one-half mile. After that, a 
dust haze persists for two to four days, and extends 
to great height. Gusty wind up to 30 knots accom¬ 
panies the harmattan, but low visibility constitutes 
its chief plague to pilots. 



95 





During the dry season, chiefly from November 
through April, the harmattan blows most con¬ 
sistently and carries much dust from the parched 
Sudan of the African interior. During the northern 


summer, when the intertropical front brings rains 
northward and a SW. monsoon wind from the Gulf 
of Guinea wets the African Sudan, the dust and haze 
belt migrates to north of io° Latitude. 


NORTH PACIFIC -S^O 


Flying weather of the subpolar North Pacific varies greatly between the seasons. In summer, fog or 
low stratiform clouds and weak winds with little cyclonic activity characterize the subpolar North 
Pacific. In winter, cyclonic activity becomes so vigorous that flying conditions change rapidly from 
the worst to the best and vice versa. During spring and autumn, flying weather fluctuates between 
summer and winter conditions, making these transitional seasons particularly undependable and per¬ 
verse. If you’re in the Aleutians in autumn, winter or spring, and don’t like the weather (which seems 
a reasonable reaction), wait a while—it will change. If you’re there in summer, try to grow fond of 
fog. The fog will wrap itself around you whether you’re fond of it or not (Figure 94). 


In subtropical portions of the North Pacific (the 
Pearl Harbor-Wake area), in contrast, seasonal 
changes are less marked and good flying weather 
prevails throughout the year. 

Understanding of North Pacific weather de¬ 
pends upon an understanding of the activity of air 
masses and fronts. 

WEATHER IN THE 

DIFFERING AIR MASSES AND FRONTS 
OF THE NORTH PACIFIC 

The prevailing patterns of air movement over the 
North Pacific (see figures 11 and 12) vary between 
winter and summer. Winter winds blow outward 
from the monsoonal Asiatic high, from the sub¬ 
tropical high (or highs) over the North Pacific, and 
from the high of the Arctic icecap with its exten¬ 
sion onto the tundras of Canada and Alaska. In the 
subpolar North Pacific, much of this air blows into 
the Aleutian low. Ascending there, it gives that vital 
area its reputation for cloudiness and rain. In the 
tropical Pacific, air enters the equatorial low or dol- 
drum belt. 

Winds of summer behave differently. In that sea¬ 
son, lows form over summer-hot Asia and North 
America and draw air from an enlarged and intensi¬ 
fied maritime subtropical high. In the average 
summer situation, air spiraling outward from this 
oceanic high dominates the ocean. In addition to 
pouring air onto the continents, it sends air south¬ 
ward to the doldrums and northward to a Bering Sea 
low-pressure trough, which is a displaced and de¬ 
cadent summer remnant of the virile Aleutian low 
of winter. 


Air Mass Weather 

The prevailing wind patterns reveal the sources of 
the principal air masses affecting North Pacific 
weather (refer again to figures 8, 9, 11, and 12). 

cP— Asiatic. In winter the monsoon winds from 
inner Asia blow with persistency and force. Gales 
frequently whip the seas off the Pacific coast to 
Siberia.* The cold air modifies only slightly in pass¬ 
ing over the cold coastal currents (Figure 10) and 
carries cold continental temperatures out to sea. It 
also carries dust. Yellow dust from inland plateaus 
and deserts—from the same earth that makes the 
Hwang Ho (Yellow River) hwang-colored—some¬ 
times settles in enormous amounts over the Yellow 
Sea and over the Sea of Japan. Lowering visibility, 
it is a second “yellow peril”. 

Cold, dry and with clear skies in its source re¬ 
gion, cP winter air from Asia slowly warms and 
picks up moisture as it moves out over the seas and 
oceans. A high percentage of cloudiness, with strato- 
cumulus clouds and ceilings of 2000 to 4000 feet, 
prevails in winter over the Sea of Japan and Kuril 
Islands (Chishima-Retto). This type of cloudiness 
continues in cP air as it moves out over the northern 
portion of the North Pacific. These strato-cumulus 
clouds are often the source of rather severe icing, 
but usually the cloud deck is thin and it is possible 
to fly over it without having to attain any great 
altitude. Also, winter ceilings are generally sufficient 
to permit flying under the cloud deck. This is a 
common practice. 

* Over the northern portion of the Japan Sea gales may be 
expected on an average of i out of 3 winter days and often 
persist for half a week. 


97 








Dutc 

Harbc 


M/DVV4Y 


98 


O 



























Fig. 93 . Locational and physical map of the North Pacific 


99 































Fig. 94. Over the top of stratus overcast near snow-capped 
Caroloi volcano in the Aleutians 


Winter cP air that flows eastward and southeast¬ 
ward from Japan arrives over the warm Japan Cur¬ 
rent. Heating rapidly from below, it absorbs great 
quantities of moisture from the warm ocean, be¬ 
comes very unstable, and builds towering cumulus 
clouds. Over the warm Japan Current it breeds the 
vigorous winter polar front of the North Pacific. 

In summer, with prevailing onshore winds, little 
cP air reaches the ocean. 

cP— Canadian. Air from Canada must buck the 
tendency to move eastward, and must cross the 
Rockies and coastal mountains, before arriving over 
Pacific shores. When cP air from Canada does over¬ 
come these barriers (and this happens occasionally 
in winter) it moves down the mountain slopes as 
adiabatically heated air—warmed, dry and stable. 
Generally this mountain-descending cP air causes 
fair weather over the ocean off American Pacific 
coasts. If originally very cold, however, it may yield 
showers of rain or sleet when it moves out over the 
ocean. 

Arctic. From the frozen Arctic and from Alas¬ 
kan winter tundras, Arctic winter air moves south¬ 
ward across Alaskan mountains and over the Bering 
Sea. Along the coast of Alaska and the Aleutian 
Island chain, this cold air reaches the relatively warm 
water of the Alaska Current. Over this relatively 
warm water the lower layers of air become unstable. 
Strato-cumulus cloud decks result, with ceilings 
usually between 1500 and 2500 feet, and with fre¬ 
quent snow flurries that reduce visibility to one to 
four miles. 


In summer, Arctic air has little force and seldom 
reaches the Aleutians. 

mP. Polar maritime air of the North Pacific win¬ 
ter originates as Arctic air or polar continental air 
over Asia or Canada. This cold continental or Arctic 
air, warming and absorbing moisture over the ocean, 
develops strato-cumulus cloud decks if warmed only 
in its lower layers. When the mP air penetrates 
southward to the Hawaiian Islands, or when it moves 
over very warm currents, heating penetrates to 
greater heights. This causes cumulus and cumulo¬ 
nimbus clouds with showery weather. 

In summer when little air of continental or Arctic 
origin moves onto the North Pacific, the subtropical 
high extends farther north and air from its northern 
side takes on mP characteristics. 

mT. About one-half of the time in winter and 
about one-fourth of the time in summer, two semi¬ 
permanent highs occupy subtropical latitudes of the 
Pacific, serving as sources of mT air masses. Most 
of the time in summer, and about one-half of the 
time in winter, one great mT source region fills this 
area. It extends farther north in summer than at other 
seasons. (Figures 8 and 9.) 

The excellent flying weather of these mT source 
regions commonly extends through the moving mT 
air masses some distance from the sources. Cloudi¬ 
ness, however, increases in mT air with increase in 
distance from the source. Stratiform clouds pre¬ 
vail to the north and east (where the air becomes 
chilled over cool sea surfaces), and cumulus and 
cumulo-nimbus clouds become increasingly numer¬ 
ous to the south and west (where the air becomes 
warmed over warm sea surfaces). In summer, when 
the general circulation pattern carries much mT air 
far north along the eastern shores of Asia, fog forms 
in the warm air over colder water, lifting to low 
stratus when strong winds blow.* These are the 
summer fogs of Kamchatka, the Kurils and the Aleu¬ 
tians. 

Frontal Weather 

Fall, winter and spring bring much vigorous 
frontal activity to the North Pacific. Summer fronts 
are scarce and weak. Figures 11 and 12 show the 
average positions of principal fronts. 

From their places of origin on the polar front 
southeast of Japan, average cyclonic storms of the 
Asiatic North Pacific winter move northeastward 
toward the Aleutian Islands, occluding as they go. 


* With an inversion 


100 


present. 




Here lies the most persistent winter polar front of 
the Pacific. Air masses that clash to produce the 
storms along this front are (i) the cold cP air masses 
from Siberia that blow as northers, and (2) the 
warmer mT air masses from the subtropical high. In 
midwinter, near the Philippines, the northers from 
the Asiatic high (cP air) and the mT air from the 
subtropical high commonly move side by side from 
the northeast (Figure 11). The somewhat colder 
northers undercut the warm moist mT air, pro¬ 
ducing clouds. Occasionally a strong surge of the 
northers, or an active push of the mT air, sets a wave 
in motion along this front, with the mT air ascending 
over the heavier cP. As this wave moves northeast¬ 
ward along the front, the moisture copiously ab¬ 
sorbed from the warm Japan Current and the increas¬ 
ing temperature contrast between the two clashing 
air masses intensify the storm activity. Sometimes 
in the winter season, and commonly in spring and 
in autumn, the mT air advances from the south along 
the Asiatic coast and meets the cP air over the East 
China Sea, or even farther north. Cyclonic waves 
then form there and move northeastward across 
Japan and on toward the Aleutians, intensifying and 
occluding as they go. In the spring (usually in 
March) when the Asiatic high begins definitely to 
break down, a second front of impressive dimensions 
frequently appears to the north of the one hereto¬ 
fore discussed. It is caused by surges of cP air from 
Siberia. Its storms move out of China or Siberia, 
cross Japan or Manchuria, unite with storms on the 
more southerly track, and advance toward the Aleu¬ 
tians.* The summer season finds the (average) polar 
front driven north of Kamchatka and the Aleutians. 
With the onslaught of autumn, polar air masses gain 
force, and, driving southeastward, again set up a 
polar front over the warm waters of the Japan 
Current. 

Along the Pacific polar front the sequence of 
events is usually as follows. Starting with the front 
over the warm Japan Current or (especially in spring 
or autumn) farther north, a series of waves develops. 
The cyclones travel eastward about 1500 miles apart 
at an average speed of about 700 miles a day 
until occluded; then they slow down, come closer 
together, and fill up. Each cyclone is followed by an 
anticyclone of cold air to the west, causing the 
polar front to be driven farther and farther south¬ 
ward and southeastward with each succeeding 


* These fronts over Asia will be more fully treated in the 
chapter on Japan and Neighboring Seas. 


cyclone. Eventually the cold air becomes heated by 
increased solar radiation, increased sea temperature, 
and subsidence. When the cold air becomes warmed, 
the polar front, then lying far to the southeast, be¬ 
comes weak and finally dissolves. The cP (norther) 
air that drove the front southeastward has been 
modified to mT and has become part of the sub¬ 
tropical high. As mT air, some of it joins the trades 
that flow equatorward, some of it returns northwest¬ 
ward to a new polar front produced by a new south¬ 
ward surge of cold cP Asiatic air. 

The polar front thus far discussed merely starts 
cyclones along their paths. Moving along this polar 
front of the Asiatic North Pacific in winter season, 
storms occlude before reaching the Aleutian Islands 
or the Gulf of Alaska. Because of its steady cyclonic 
(counterclockwise) circulation, the Aleutian low 
becomes a focal center, or a gathering point, for 
cyclones. The occluded fronts move around its 
southern side like spokes of a wheel. This frontal 
movement is limited to the southern side of the Aleu¬ 
tian low because mountains and the North American 
winter high-pressure center prevent fronts from 
passing northward through Alaska. 

When cold season cyclones reach the Aleutians 
and the Gulf of Alaska, cold continental winds from 
the Arctic feed them from the north, and cool mari¬ 
time (mP) winds from the ocean feed them from 
the south. Here, where Arctic air moves to meet 
maritime air over relatively warm water, is the 
Pacific Arctic front of winter (Figure 11). Though 
many occluded storms dissipate in the Gulf of 
Alaska, others become strongly regenerated, with 
waves developing on the occluded fronts. 

From the Gulf of Alaska, cyclones (generally oc¬ 
cluded) attempt to cross the mountains into Canada 
and northwestern United States. They cross the 
coastal ranges slowly and produce disagreeable flying 
weather for long periods. Most of them stagnate 
along the Rockies, their mild and moist mP air un¬ 
able to displace the colder cP air commonly over 
North America in winter. 

When the Pacific subtropical high divides into 
two cells or segments (as it does 50% of the time in 
winter and 25% of the time in summer) a front 
forms in the vicinity of the Hawaiian Islands (Figure 
11). Along this front the kona storms develop, and 
move northeastward. Those that succeed in moving 
beyond the realm of the NE. trades, which stunt 
them, may develop quite vigorously and advance to 
the American coast, generally occluding against the 
mountains. 


101 





Nome 


WINTER WINDS 

SURFACE and ALOFT 

For each station ■■ 

Lower wind rose represents SURFACE winds. 
Upper wind rose represents winds ALOFT 
/ at 6000 ft 


Dutch Harbor 


Kanaga 
island * 


SCALE : 

PERCENTAGE OF OBSERVATIONS IN 
WHICH WinD BLOWS FROM EACH 
DIRECTION is shown so 30 10 % 

by length of line: I' I' I' f ’ I ' C 

a r\ r>t\ no* 


harbor 


SUMMER WINDS 

SURFACE and ALOFT 

For each station: 

Lower wind rose represents SURFACE winds. 
Upper wind rose represents winds ALOFT 
at 60(?P Ft 


• Nome 


Dutch Harbor 


ttariaga 
'stand • 


Midway . 


SCALE : 

PERCENTAGE OF OBSERVATIONS IN 
WHICH WIND BLOWS FROM EACH 
DIRECTION is shown 50 30 10 % 

by length of line: h ~ hH" I 'O 


102 


Fig. 96 






















































Fig. 97 



Fig. 98 


103 

















































WEATHER HAZARDS AND HELPS TO AVIATION— 

THE GENERAL PATTERN 

Flying on missions over the North Pacific, the 
Navy pilot needs to know about: 

Winds .Direction and force; surface and aloft. 

Icing Zones ....Dependent on temperatures and on 
presence of clouds or precipitation. 

Visibility .Both horizontal visibility and ceiling 

(vertical visibility). 

The pilot needs this information for the time a?id 
route of each particular flight. A knowledge of the 
average patterns of these helps and hazards (as pre¬ 
sented in this section) aids the pilot to interpret 
correctly available weather information, fill in the 
gaps with reasonable conjectures, understand the 
weather through which he flies, and know which 
way to turn for an “out” when he gets into some¬ 
thing too tough to fly through. 

Winds 

Surface Wind—Directions. Figures 8 and 9 
show the prevailing surface wind directions, with 
(generalized) constancy and force indicated. Figures 
95 and 96 give a more detailed picture of average 
winds at key points. They show two wind roses for 
each selected station, with the lower rose indicating 
surface winds, and the upper rose indicating winds at 
the 6000 feet level. To fix firmly in mind the surface 
wind probabilities of different parts of the North 
Pacific, relate Figures 95 and 96 to Figures 8 and 9. 

Note, for example, that winter surface winds at 
Tokyo blow dominantly from a northerly or 
westerly direction, while most summer winds blow 
from a southerly or easterly direction. Figure 95 
tells the same story as Figure 8. 

Wake Island and Pearl Harbor, well within the 
trades, have prevailing easterly winds at both seasons. 
Midway, swept in winter by prevailing westerlies 



Fig. 99. Wind and snow in the Aleutians (U.S. Navy photo) 

104 


with much frontal activity, is definitely a trade wind 
station during summer, when the subtropical high 
grows most powerful. 

Winter surface winds at Juneau, at Nome, and 
in the Aleutians, normally spiral counterclockwise 
around the Aleutian low, but feel the disruptive in¬ 
fluence of many frontal movements. In summer, 
with weakening of the Aleutian low, winds of this 
area show less definite pattern, though southerly 
winds bring frequent fogs to the Aleutians. 

Along the California coast, northwest winds pre¬ 
vail throughout the year. Farther north, Seattle ex¬ 
periences dominantly southeast surface winds in 
winter, and northwest winds in summer, when the 
subtropical high moves northward. 

Throughout the entire North Pacific, as in rhe 
North Atlantic, areas with the most frontal activity 
have the most variable winds. 

Wind Aloft—Directions. At 6000 feet eleva¬ 
tion, average wind directions remain about the same 
as at the surface (Figures 95 and 96). And at many 
stations they are fully as variable as the surface winds. 

If you fly higher, wind directions become more 
dependable. Above 10,000 feet, west winds of re¬ 
markable reliability prevail over the entire North 
Pacific from 20 °N. Latitude poleward. 

Wind Force. Wind force or velocity averages 
somewhat greater in winter than in summer, and 
increases with increase in altitude. Winter gales fre¬ 
quently whip the Pacific in the broad belt between 
Kamchatka and Midway (Figure 97). Summer gales, 
much less common, occur chiefly in the Bering Sea 
and the Aleutian area (Fig. 98). 

Icing 

In the northern portion of the North Pacific, icing 
problems confront pilots at all seasons. The danger is 
greatest in the fall, winter, and spring—whenever 
surface temperatures approach freezing (Figure 100). 
Icing may then be encountered in the low cloud 
decks so characteristic of the area, and in precipita¬ 
tion. In middle latitudes over the ocean, icing may 
occur near the surface when and where winter cold 
fronts bring freezing or near-freezing temperatures. 

When surface temperatures are not close to freez¬ 
ing, the height at which ice will probably be en¬ 
countered may be estimated by allowing for a tem¬ 
perature decrease of 3°F. per 1000 feet. Where win¬ 
ter surface temperatures average 50°F., for example 
(that is, along the 50° F. isotherm that runs from 
south of Japan to the Oregon coast in Figure 100), 
icing normally may be encountered wherever a plane 






w TEMPERATURES of AUGUST 


GUI? 
Of u 
At,* 5 * 




Fig. 101 






















































flies through clouds above 5000 or 6000 feet. Air 
mass movements, of course, greatly alter the average 
winter temperatures. 


In summer you’ll be in icing danger over the Aleu¬ 
tians whenever you fly in clouds above about 5000 
or 6000 feet. Southward over the ocean the average 
level of icing becomes higher (Figure 101). 

Icing in the North Pacific will most frequently 
occur in connection with the following situations: 
frontal activity, instability showers, stratus and 
strato-cumulus decks, clouds along mountains, 
“Arctic sea smoke”, and freezing spray. 

You know the normal locations of the frontal 
zones. 

Instability showers occur in the northern part of 
the ocean in fresh polar outbreaks of the colder 
seasons. 

In the subpolar North Pacific, icing in stratus and 
strato-cumulus may be experienced at any time of 
the year—and may be a problem for days at a time. 
In the Aleutians the strato-cumulus decks generally 
have ceilings of about 1500 feet and thicknesses of 
about 3000 feet. In summer in this area, then, it is 
possible to get above them with little icing, but in 
winter these clouds contain maximum icing danger. 
In autumn, winter, and spring, pilots on mission in 
the Aleutians should not fly for any great length of 
time in stratus or strato-cumulus clouds. 

Clouds along mountains of the Pacific coast and 
Aleutians commonly contain the greatest possible 
icing hazard. Temperatures of the American Pacific 
coast are much milder than those of the Asiatic 
coast (Figs. 100 and 101), owing chiefly to the pre¬ 
vailing westerlies (the westward movement of air 
masses). From northern California northward, how¬ 


ever, clouds against the mountains (and clouds in 
frontal formations stagnating along the mountain 
ranges) may contain icing. This is a common hazard 
along Alaskan coasts even in summer. In colder 
seasons, the clouds and precipitation caused where 
west winds rise over mountains, and the clouds and 
precipitation caused by the abundance of cyclonic 
activity in the Gulf of Alaska and southward along 
the coast, make flying missions in this region experi¬ 
ences in outwitting ice. 

“Arctic sea smoke” is a steam fog. You’ll often 
see it as a light, wispy fog that rises ’from the water 
when cold air moves over warmer water (Fig. 102). 
When you take off or land through ‘this stuff, hoar 
frost forms on wings and windows. This relatively 
innocent nuisance has a dastardly relative, also a 
member of the “Arctic sea smoke” family. Under 
certain conditions, as when very cold air lies over a 
rift in pack-ice, cumulus clouds billow upward 
from the sea surface. These clouds are loaded -with 
water droplets that are aching to become wing ice. 

Freezing spray can also be a nuisance. When air 
temperature is below freezing and your seaplane is 
cold, spray will ice up the plane. Under those condi¬ 
tions, don’t taxi on the water any more than abso¬ 
lutely necessary. 



Fig. 102. Steam fog (U.S. Navy photo) 


106 







Visibility 

South of 35°N. Latitude, in the region of the sub¬ 
tropical high and trade winds, visibility offers minor 
problems by comparison with problems farther 
north. They are: (i) along California’s coast, fre¬ 
quent fogs cloak cold coastal waters; (2) over warm 
seas, precipitation lowers visibility in the sometimes 
torrential instability showers; (3) along mountains 
standing in the trade winds, clouds may hide peaks; 
(4) when winter fronts invade these subtropical 
latitudes, frontal rainfall and fog can limit visibility; 
and (5) over most subtropical waters, haze is a com¬ 
mon pest. Compared to the visibility difficulties of 
more northern latitudes, however, the commonly 
clear horizons of subtropical skies make this a pilot’s 
heaven. 

Purgatory lies farther north. Fog and falling or 
blowing precipitation, both air mass and frontal, 
place the Aleutian and Kuril island-stepping-stones- 
to-Japan among the least negotiable islands in the 
world. The ocean area south of Kamchatka shares 
this distinction of sporting some of earth’s “thickest” 
weather. Pea-soup fog or blowing snow or rain 
often hide the sea off Alaska—and more often hide 
the coast. In winter, frontal precipitation and frontal 
fog account for most of the visibility restrictions. 
In summer, the northward sweep of warm (mT) air 
over cold sea surface produces advection sea fog. 
Notice the fog maps (Figures 103 and 104). Based 
on observations made at 1200 G.C.T., these fog maps 
show night conditions over the Pacific. Advection 
fog over the sea, however, is no great respecter of 
clocks. When warm moist air moves over cold water, 
fog lasts night and day. 

North of the Aleutians and the Kurils, over the 
Bering Sea and the Sea of Okhotsk, pilots encounter 
two new visibility hazards: (1) “Arctic sea smoke” 
which you’ve learned as also an icing hazard; and 
(2) the deceptive depth perception— common when 
these seas are frozen in winter—that pilots often 
experience when flying the Arctic airways. Concern¬ 
ing the later, some pilots find that under certain 
atmospheric conditions it’s absolutely impossible to 
distinguish the sky from the terrain—it’s like flying 
in a bowl of milk. Some Arctic tragedies have re¬ 
sulted from pilots placing too much faith in their 
depth perception.* One patrol plane actually flew 
to a safe landing on an ice cap when the pilot didn’t 
know that he was anywhere close to the surface. 
One way to be sure where an ice or snow surface is: 


* Rear Admiral Richard E. Byrd considers this the greatest 
hazard you can encounter in Arctic flying. 



drop a black object from the plane—where it stops, 
that’s the surface. 


FLYING WEATHER OF SPECIAL AREAS 
The Pacific Coast of North America—from San Diego to Kodiak 

If you fly from San Diego to Kodiak you’ll pass 
from subtropical to subpolar marine climate—from 
the good flying weather at the edge of the subtropical 
high to the ever-changing “weather factory” of the 
Aleutian low. You’ll fly through three types of cli¬ 
mate: (1) subtropical “Mediterranean ” climate of 
southern and central California, with its warm dry 
summers and cool moist winters; (2) the temperate 
marine climate of the Pacific Northwest and British 
Columbia, with some characteristics of the climate of 
southern and central California, and some character¬ 
istics of the climate of the Pacific coasts of Alaska; 
and (3) the subpolar marine climate (cold and wet) 
of the Gulf of Alaska and its coasts. 

The four climatic controls (latitude, land and 
water distribution, ocean currents, and topography) 
all make marked impression on the climate of this 
long coast. 


107 







108 










































160 ° 


Fig. 105. Sea temperatures along the 



AUGUST 


Pacific coast of North America 


109 





















In these subtropical to subpolar latitudes on the 
west coast of a continent, maritime air masses domi¬ 
nate. During both summer and winter, mP and mT 
air masses, moving to these coastal areas from a 
(general) westerly direction, produce most of the 
flying weather from San Diego to Kodiak (Figures 
ii and 12). Cold ocean currents and upwelling of 
cold water along the California coast, and relatively 
warm ocean currents to the north (Figures 10 and 
105), tend to produce opposite effects on the air 
masses that cross them. Along the California coast 
the relatively cold water tends to make the air stable, 
the clouds stratiform. Farther north the relatively 
warm water produces more instability, more cumuli- 
form clouds. High mountain barriers (Figure 93) 
cause orographic cloudiness and precipitation and 
retard movements of fronts. They also largely shut 
off the influence of the continent. cP air must cross 
these mountains (and in addition, it must overcome 
the tendency to move eastward) before it can reach 
the Pacific coast. During the winter season, however, 
much Arctic air surges onto the Gulf of Alaska from 
the Bering Sea, and some Arctic and cP air crosses 
the mountains from Alaska and Canada. 

Fronts of this area are more active in winter than 
in summer. 

In summer, the strengthened subtropical high 
shifts poleward and the polar front moves far north 
(Figure 12). Cold surges of Arctic air cause infre¬ 
quent weak polar front cyclones along the Alaska 
coast. These depressions travel from the Aleutian 
region a short distance along the Pacific coast; so 
that only a few mild cyclones reach northwestern 
United States, and southern California has no mov¬ 
ing weather in summer. 

In winter, cyclones that advance along the prin¬ 
cipal polar front of the Pacific (Figure 11) com¬ 
monly occlude near the Aleutians or over the Gulf 
of Alaska. Here the cyclones may dissipate, or they 
may move eastward as occluded fronts and linger for 
several days along the coast of Alaska and British 
Columbia. 

The front that most actively influences the weather 
of the Pacific coast is the secondary polar front of 
the Pacific. The air masses converging along this 
front are maritime Pacific (mP) and maritime tropical 
(mT). Flowing roughly parallel from the southwest 
(Fig. 11), these air streams develop waves on the 
front when there is a marked difference in their 
relative velocity. Once a wave is initiated, a “family” 
of waves develops, and the mean position of the 
110 


secondary polar front is shifted southeastward until 
it finally dissolves. The first wave of a family usually 
develops a cyclone that brings widespread precipita¬ 
tion to southern Alaska stations. Succeeding waves 
reach farther and farther south along the coast, until 
the last member of a family affects coastal weather 
as far south as southern California. 

This frontal activity helps explain the north to 
south decrease in cloudiness and precipitation along 
the Pacific coast. Northern sections have frequent 
disturbances, while southern California storms occur 
at intervals of approximately 7 to 10 days. In north¬ 
ern California, Oregon and Washigton storms are 
often preceded by several days of cloudiness. This 
is because each of the early waves of a “family”, 
which may cause stormy weather to the north, gives 
rise only to cloudy weather at a locality such as 
Portland, Oregon. 

The rather excessive cloudiness and heavy precipi¬ 
tation that you’ll find in winter along the coast north 
of Latitude 40 0 is due to frequent cyclones, but it is 
also due to lingering warm front occlusions that 
characterize that coast. In winter the land surface is 
colder than the water surface, so that a cold wedge 
of air lies against the mountainous coast most of the 
time. As fronts approach the coast they occlude— 
the invading mP air, modified by contact with warm 
water, rises over the cold wedge of coastal air (Fig¬ 
ure 106). Persistent cloudiness and torrential rains 
result, with tremendous icing hazard. This condi¬ 
tion may last for several days, as a “family” of 
cyclones advances and one cold front after another 
passes over the wedge of cold coastal air. 

The Arctic front adds to the storminess of the 
Gulf of Alaska, and occasionally (more frequently 
some years than others) storms on this front push 
southward, affecting the weather of the west coasts 
of Canada and United States. These storms are 
notable for the cold weather they bring, for the air 
behind the cold fronts of the cyclones is Arctic 
continental. 


WINTER SEASON 



Fig. 106. An occlusion along coastal mountains 











Flying Weather of the Southern and Central 
California Coast. In summer this subtropical coast 
lies under an edge of the subtropical high. Almost no 
frontal disturbances invade the region, and stable air 
masses drift from the ocean, across cold coastal 
waters, onto warm land (pp. u, 18). At sea, then, 
flying conditions are excellent, with patches of strati¬ 
form clouds. Over the cold coastal waters, fogs and 
low stratus provide the chief summer hazard to avia¬ 
tion. Over the land, the sun and stars reign unchal¬ 
lenged, except locally when fogs roll in from the sea. 

The rare “cloudbursts” that hit northern Mexico 
and southern California in summer are caused by mT 
air from the Gulf of Mexico, either at the surface 
or aloft. This air brings not only thunderstorms 
but also oppressive (humid) heat. Californians call 
it “Sonora” weather—they also call it “unusual”! 

In winter, the southern and central California 
coast has much good flying weather, but this season 
brings far more trouble than you’ll find in summer. 
Occluded cyclones at intervals of 7 to 10 days bring 
rain, squalls, lowered ceilings and poor visibility. 
Coastal fogs are slightly less common than in sum¬ 
mer, but maritime air masses, stagnating over interior 
valleys, cause persistent valley fogs that may halt air 
operations. When flying over the ocean you’ll find 
only patches of strato-cumulus clouds, except in 
the presence of occasional cyclones. 

Cloudiness and Ceilings. The maritime air be¬ 
comes stabilized over cold water before reaching the 
coasts. Most clouds are therefore stratiform—stratus 
in summer, strato-cumulus in winter. Ceilings aver¬ 
age 700 feet under the summer stratus, 1000 feet 
under the winter strato-cumulus. But clouds are not 
always present, so ceilings remain below 1000 feet 
only 15% to 30% of the time in summer and only 
6% to 12% of the time in winter. 

Visibility. In winter you'll find visibility less than 
one mile only 2% to 5% of the time, in summer 
only 1% of the time. 

Fog causes much of this low visibility. It occurs 
on California coasts 4% to 7% of the time in winter, 
5 % to 12 % of the time in summer. This coastal fog, 
chiefly a morning phenomenon, usually “burns off” 
by 1000. The interior valley radiation fogs of win¬ 
ter, however, may persist for days (Fig. 107). 

Precipitation, which also lowers visibility, is dis¬ 
tinctly seasonal, occuring in winter. Dull gray days 
with long, continued rain may occur, but showers of 
short duration are more common. Even the rainiest 
months of winter usually have less than a dozen rainy 
days, and rains seldom obstruct operations. 



Fig. 107. Advection fog over the Southern California 
coast, cirro-stratus above (U.S. Navy photo) 


Hazy air may be encountered in southern Cali¬ 
fornia in summer. 

Icing is no problem. 

Winds. The westerly to northwesterly surface 
winds (Figures 8 and 9) average 15 to 30 knots. 
Above 6000 feet, northerly winds prevail, and at 
25,000 feet westerly winds blow at a velocity of 40 
knots or more. 

Flying Weather of the Pacific Northwest 
and British Columbia Coasts. In summer the 
northeast quadrant of the subtropical high, which 
extends farthest north at this season, dominates 
the air circulation of this central section of the Pacific 
coast (Figure 9). Maritime air moves from the west 
and northwest over coastal waters that are relatively 
cold to the south, relatively warm to the north 
(Figure 105). A few frontal disturbances affect the 
British Columbia coast in summer, but almost none 
reach northern California. Northern California, 
Oregon and Washington, then, have summer flying 
weather resembling that of southern and central Cali¬ 
fornia-excellent over the ocean and over the land, 
but with much fog and low stratus cloaking cold 
coastal waters. To the north, along British Columbia, 
summer flying weather remains good, but it’s slightly 
different. Owing to increased frontal activity to the 
north, cloudiness increases; and owing to change 
from relatively cold to relatively warm coastal 
water, coastal clouds change from dominantly strati¬ 
form to cumuliform. 

In winter, maritime air chiefly from the south and 
southwest (circulating around the southeast quad¬ 
rant of the Aleutian low) dominates this region. 
This martitime air, chilling over cool waters and cold 


111 



land, drapes ocean, coasts and land with many strati¬ 
form clouds and fogs. When this moist air stagnates 
over the land it produces persistent radiation fogs in 
interior valleys. Frontal storms move in from the 
northwest, increasing the prevalent low cloudiness 
and fogs and producing heavy precipitation. Surges 
of continental air bring occasional spells of clear 
weather over the land and produce cumulus clouds 
over the sea. 

Cloudiness and Ceilings. Cloudiness is much greater 
here than in southern and central California. It aver¬ 
ages about six-tenths in winter and about four- 
tenths in summer. At Seattle, ceilings below 1000 
feet occur 14% of the time in autumn and winter 
(September to February), and only 3% of the time 
during the rest of the year. 

Visibility. In general, visibility is good in sum¬ 
mer, poor in winter. In winter you’ll find visibility 
less than one mile about 10% of the time, in summer 
only 2% or 3% of the time. 

Fog causes much of this low visibility. Seattle has 
fogs on over 20% of the observations in winter, only 
2 % in summer. On the coasts, however, late summer 
and autumn is the foggiest period, but you can expect 
coastal fogs at any season—and northern California 
has the foggiest section of the Pacific coast. 

Precipitation is heaviest during winter, scant in 
summer. Winter precipitation is extremely heavy on 
the windward slopes of coastal mountains, often 
occuring as wet, heavy snow that reduces visibility 
to near zero. 

Icing rivals poor visibility as a flight hazard in this 
region. With winter surface temperatures around 
40°F. (Figure 100), and a high degree of cloudiness, 
you may acquire ice from 2000 to 8000 feet in stratus 
clouds and from 2000 to 13,000 feet in the turbulent 
clouds over mountains.* You’ll find icing at its worst 
over the mountains and in occlusions. 

Summer presents little icing danger (Figure 101). 

Winds of winter sometimes attain hurricane force 
(65 knots) on the ocean coasts; and even inland val¬ 
leys and protected sounds have, at times each winter, 
sufficiently severe winds to damage aircraft. The 
winter winds, however, average only 15 to 30 knots; 
and summer winds are quite docile. Winter winds 
are dominantly southerly, and summer winds prevail 
from the northwest, but mountainous topography 
and cyclonic activity (the latter chiefly in winter) 
cause much variation in surface wind directions. 
Winds aloft attain greater force; but, up to over 

* Compare this with icing levels along the shores of north¬ 
western Europe. 


6000 feet, they are only slightly more reliable as to 
direction (Figures 95 and 96). 



Fig. 108. Ships and a storm visit Kuluk Bay, at Adak 
in the Aleutians (U.S. Navy photo) 


Flying Weather of the Gulf of Alaska and 
Its Coasts. In summer, moist maritime air sweeps 
from the south, chills over these northern waters and 
coasts, and forms low stratus clouds and fogs. Gen¬ 
erally you’ll find this summer air smooth, but with 
poor visibility except on top. Occasionally, how¬ 
ever, surges of Arctic air produce frontal storms 
even in summer. In Arctic air, back of the fronts, 
flying weather is good—in summer. 

Winter has much more frontal activity. Occluded 
polar cyclones often lie over the Gulf of Alaska or 
linger along the coast. When the Arctic front 
surges onto the sea, storms advance in rapid succes¬ 
sion, usually to occlude against the coast of Alaska, 
Canada or the United States. When mT air moves 
far north, it brings fog and low stratus; when Arctic 
or cP air rides onto the relatively warm Gulf, it 
brings gales, turbulence and cumuliform clouds; in 
the dominant mP air masses, strato-cumulus decks 
drift from the west and may boil up into towering 
cumulus along the coastal mountains. Icing in these 
many clouds is the greatest hazard of winter. 

Cloudiness and Ceilings. The sky averages about 
seven- or eight-tenths obscured, with summer slightly 
cloudier than winter. Along the coasts, onshore 
winds favor low ceilings, while offshore winds favor 
cumulus clouds with higher ceilings. You’ll find 
ceilings below 1000 feet about 40% of the time at 
Kodiak, 25% of the time at Sitka. 


112 



Visibility. Falling rain or snow is the chief cause 
of low visibility. Cordova, on an exposed mountain¬ 
ous coast of the Gulf of Alaska, has 146 inches of 
precipitation annually, and many coastal points of 
southern Alaska have more. Winter, with its exces¬ 
sive frontal activity, brings more precipitation than 
summer. 

Fogs are more prevalent in summer, when warm 
air from the south chills over cold water. Cordova 
has fog 14% of the time, Sitka 6%, Kodiak 5%.* 

Icing. In winter, when surface temperatures aver¬ 
age near freezing (Figure 100), icing may be en¬ 
countered from the surface to anywhere from 6000 
to about 11,000 feet. In summer, when surface tem¬ 
peratures average nearly 55 °F. (Figure 101), you’ll 
find icing where clouds extend above about 6000 
feet. 

Winds. In this area, wind directions are extremely 
variable, both at surface and aloft (Figures 95 and 
96). Gales often whip these seas and coasts—20 times 
a year at Kodiak, chiefly in winter. And in this re¬ 
gion you’ll find much coastal (mountain) turbulence, 
and you’ll meet the voilli'wa'w , to be discussed in the 
section on the Aleutians. 

The Aleutian Islands 

Cloudiness and Ceilings. You’ll suffer little sun¬ 
burn in the Aleutians. Cloudiness prevails. Periods of 
clear weather are of short duration. Though there is 
much cloudiness near Kamchatka and along the 
Alaska coast, the central Aleutians are cloudiest. 
This increase in cloudiness with distance from the 
continents is what you should normally expect, for, 
when cP air blows from cold land onto cool sea that 
is warmer than the land, the air remains clear over 
the land but forms low stratus or strato-cumulus 
clouds over the sea. The low stratus may at times 
build down close to the sea surface—and pilots, 


* These figures, like the 12% for Seattle, represent the per cent 
of total observations in which fog was reported. 



— SUCH CLOUDS ARE ALWAYS STUFFED WITH ROCKS. 


knowing that clouds can hide mountain sides, fly 
low over the waves, or they fly on top. 

Air from the continents forms cumuliform clouds 
at all seasons, but such air is common over the Aleu¬ 
tians only in the winter half-year. In summer, air 
masses originating in the subtropical high often move 
to the Aleutians as southerly winds. Under this com¬ 
mon summer situation the cloudiness continues, and 
visibility becomes an even more acute problem. 
When dense fogs aren’t hugging the summer sea 
(Figure 104), it is often because strong winds have 
lifted them into low stratus. Skies seldom clear. 

.An overcast covers the central Aleutians more 
than 60% of the time, and unlimited ceilings occur 
only about one day in ten. In the western Aleutians 
near Kamchatka, and along the Alaskan coast, un¬ 
limited ceilings occur more frequently—from two to 
five days in ten. Dense fogs, however, bring both 
the ceiling and visibility to zero on many summer 
days, especially in the western Aleutians and off the 
Kamchatka coast (Figure 104). 

Visibility. Don’t expect the summer fog to be 
wispy and ghost-like. It’s of the advection variety 
and extends in solid layers that may be a few feet 
thick or as much as 4000 feet thick. It “lies down” 
on the region and sometimes even winds of gale force 
can’t lift it. 

The windward side of an island is foggy more 
often than the leeward side. Remember these three 
possible situations: 

(1) A shallow advection fog may flow around an 
island, leaving a clear space on the leeward side 
(Figure 109). 

(2) A thick fog may flow over (as well as around) 
an island, but the adiabatic heating produced by the 
downdraft on the lee side of the island causes the 
fog to lift to low stratus, and causes breaks in the 
low stratus (Figure no). 

(3) The fog flowing over an island may be so 
thick that the adiabatic heating on the lee side merely 
lifts the fog to low stratus and cannot break the low 
stratus (Figure m). 

Even on the foggiest days of summer there is a 
good possibility that on leeward sides of mountain¬ 
ous islands you’ll find either clear areas or breaks in 
the overcast. If neither is present, there’ll probably 
be a ceiling (lifting of fog) on lee sides. 

Low visibilty in the Aleutians cannot be asso¬ 
ciated entirely with advection fog. Other types of 
fog, and blowing and falling snow and rain, also hide 
landing fields. Like the summer advection fogs, most 
of these visibility hazards are worse on windward 
coasts. 


113 





Fig. 109. A shallow fog, flowing around an island, may leave 
a clear space on the leeward side 

Autumn and winter have the maximum precipita¬ 
tion, for then many cyclonic tracks lead over the 
Aleutians and the cold air absorbs much moisture 
from the relatively warm water. Therefore, drifting 
and falling snow cause much of the low visibility in 
autumn and winter. Frontal fogs occur at all seasons. 
Radiation fog can become a nuisance at inland valley 
bases, chiefly in winter, and “Arctic sea smoke” is 
another winter hazard. 

These conditions combine to give most of the 
Aleutians poorest visibility in summer and winter. 
Flying conditions are better, so far as visibility is 
concerned, in autumn and spring, with conditions 
better in the autumn than in the spring, for the 
advection fog of summer starts in the spring (Figure 
112). 

Northward over the Bering Sea, visibility becomes 
even worse than in the Aleutians, owing chiefly to 
falling and blowing rain and snow—and, on occa¬ 
sion to “Arctic sea smoke”. Westward toward the 
Kurils, winter visibility improves, but summer fogs 
often blot out these seaways toward Japan. Along 
the coast of Alaska visibility is better than in the 
Aleutians and it becomes increasingly better over 
the ocean away from the coast. 

In all the region severe gusts often sweep down 
the mountains and into the sea, raising spray and 
rapidly reducing visibility to as little as 50 feet. These 
willi'waws (to be discussed later) are even more dis¬ 
astrous because of their high wind velocities and 
gusty unpredictability. 



Fig. no. A thick fog, flowing over an island, may leave 
breaks in the overcast on the leeward side 


Salt haze is common in the Aleutian region, and 
can be effective in reducing the visual range. 

Mirages (or “looming” if you must be technical) 
add an interesting touch to the visibility problem. 
With calm sea and a shallow temperature inversion, 
you may see’ objects that actually lie below the 
horizon. 

Icing danger is great in the cold season, moderate 
in summer. 

In the cold season, with surface temperatures near 
freezing, you’ll find icing danger in precipitation, in 
the prevalent low cloud decks, and in occasional in¬ 
stability showers. It is often wiser to fly over, under, 
or around clouds than through them. Fortunately, 
the winter seas are seldom fogged, and this permits 
you to fly under the cloud deck. Or, if not on a 
patrol mission, you can fly over the clouds, for the 
Aleutian cloud decks seldom extend above 7000 
feet. Before taking off in the colder spells of winter, 
you should make sure that your plane hasn’t a load 



Fig. in. A very thick fog, flowing over an island, lifts 
to low stratus on the leeward side 


of frost or wing ice. If it has, use a stiff corn broom. 
If flying a seaplane, avoid freezing spray as much 
as possible. 

In summer, ice may be acquired in clouds above 
5000 or 6000 feet, but few clouds extend above that 
level. You will seldom encounter icing in summer fog 
because surface temperatures are usually several de¬ 
grees above freezing. 

Winds. Average winds of the Aleutians have been 
discussed on page 104 above. Wind directions are 
most dependable in summer and winter, for in sum¬ 
mer the mean path of polar front cyclones lies north 
of the Aleutians (Figure 12), and in winter it lies 
to the south (Figure 11). In spring and fall most of 
the storms move directly through the Aleutians, 
causing extremely variable winds and variable 
weather. 

Beware of voillivoaves. They’re terrific! They are 
sharp-edged gusts of wind with velocities which may 
reach 100 knots or more, and with vertical currents 


114 

















W/U/IA/AWS ARE CHA/NED 

TO THE LEEWARD S/DES OFMOOAITA/N 5 
that may throw your plane around so roughly that 
crew members are knocked unconscious. (This has 
happened.) Williwaws occur in the area of disturbed 
air currents on the lee side of mountainous islands. 
Their exact position can’t be fixed because it depends 
upon the contours of the land and the strength and 
direction of the wind. Williwaws have one redeem¬ 
ing trait—they won’t chase you. They’re chained to 
the land. The only way you can get into trouble with 
one is to fly into it. Don’t. Instead, when a strong 
wind blows, you can choose tracks most likely to 
avoid great turbulence, staying away from sharp 
cliffs, bluffs, peaks and valleys. If visibility is suffi¬ 
cient on the windward side of an island, it’s some¬ 
times a good idea to fly on that side, because turbu¬ 
lence is less there. If you must climb to “top” hills 
from the lee side, gain your altitude before reaching 
the area of downdrafts. When you must fly on a 
lee side while strong winds blow, maintain sufficient 
altitude to permit recovery from any stray down¬ 



Fig. 112. Upslope fog, banner cloud, and strato-cumulus on 
the mountains in the Aleutians (U.S. Army photo) 


draft. And don’t bank sharply in changing course 
near land; an unforeseen williwaw may spill you. 

Seasonal Summary. Best flying conditions in the 
Aleutians occur in the spring and fall months. Con¬ 
ditions in winter are very poor because of severe 
icing, frequent gales, mechanical turbulence where 
strong winds blow over mountains, and low clouds 
with much snow. Conditions in summer are poor be¬ 
cause of the frequency of fogs and low stratus clouds. 
Conditions in spring and fall are not exactly a pilot’s 
paradise, but these seasons are better than winter and 
summer. The extreme changeability of the cyclonic 
weather of spring and fall provides spells of good 
flying. 

Contact or on Top? Aleutian weather may look 
tough, but it’s only skin deep! There’s generally a 
pronounced temperature inversion in this region— 
that is, it’s warm upstairs. Much more significant, 
the clouds seldom extend above 7000 feet. Pilots 
prefer to fly contact, and they must fly con¬ 
tact on most missions. But if the weather closes in 
on you, it’s generally possible to go on top, spend¬ 
ing a minimum of time in the cloud if it carries ice 
(Fig. 113). 

When you break out above the overcast, of course, 
you’ll still need to think about “the way to go home”. 
Variable winds above the overcast help you to lose 
yourself, and variable pressures give your altimeter 
funny ideas. If the clouds aren’t too thick you can 
orient yourself by the Aleutian peaks—if you know 
the peaks. You probably won’t know them, however. 



Fig. 113. Patrol planes above strato-cumulus over Kiska 
(U.S. Navy photo) 


115 





until you get into the region and study its geography. 
It’s a rough terrain, not properly co-operative with 
pilots, except that it provides landmarks—a series 
of rocky promontories that stick out of the sea with 
saw-toothed edges. Explore your neighborhood in 
fair weather and frequently stick your nose into 
maps. It’s inadvisable to mistake two headlands of a 
bay for two islands, or to mistake Mt. Klyucherskaya 
for Attu. It’s equally inadvisable to “let down” into 
a mountainside. Fortunately, the Aleutians have as 
good radio navigation as any part of the world. Even 
if your geography fails completely, you generally 
can come in on the beam. 

One More Caution. Aleutian and Alaskan val¬ 
leys and lowlands are covered with tundra. Tundra 
may look like a marvelous landing field, but it’s a 
matted, tangled growth that will throw you for a 
loss. For a forced landing, select a hard sand beach 
at low tide—if it’s low tide, and if you can find a 
hard sand beach. 

Pearl Harbor-Midway-Wake Area 

On missions in the mid-Pacific—in the environs of 
Pearl Harbor, Midway and Wake—pilots find com¬ 
paratively friendly weather. During the summer sea¬ 
son nearly all flying in this area is done in mT air— 
that is, in the subtropical high pressure areas and in 
mT air blowing outward from them, or in broad 
cols or narrow troughs between two high-pressure 
areas. In the cooler seasons, also, much flying is done 
in mT air. In fall, winter and spring, however, many 
vigorous polar fronts reach these seas. The outbreaks 
of polar air, and the consequent polar frontal activity, 
make the winter weather more variable than that of 
summer. Chapter 6 will more fully treat flying 
weather of the subtropical Pacific, but the following 
tables and illustrations summarize characteristic fly¬ 
ing conditions within mT air, polar air, and polar 
fronts. 

116 


Flying Conditions in mT Air of the Subtrop¬ 
ical Pacific: 


Sky .Two- to five-tenths cloud covei. 

Cloud types .Dominantly cumulus (ragged 

looking). 

Ceilings .2000 feet average. 

Cloud tops .8000 feet average. 

Precipitation....Scant; in instability showers. 

Turbulence.None in clear air, except below 


cloud bases. 

Slight in clouds with tops below 
8000 feet. 

Severe in clouds with tops at 
12,000 feet or higher. 


Visibility.Hazy below 8000 feet. 

Icing level.16,000 feet average. 

Winds .Surface winds are anticyclonic, 

blowing outward from the high 
pressure areas; above 8000 feet 
the winds aloft are westerly. 

Special features.Some cumulo-nimbus clouds. 

Avoid them if they extend above 
8000 feet. Cols, or broad areas 
between two cells of subtropical 
high. They have stratus clouds 


and some scattered cumulus that 
build up to 12,000 feet. In the 
cols the westerly winds aloft ex¬ 
tend down to 6000 feet. 

Troughs, or narrow north-south 
belts of convergence between 
southeast and northeast winds 
from adjoining subtropical high 
pressure cells. Along a trough 
cumulus clouds build up to 10,000 
feet, and eastward from a trough 
broken alto-cumulus sheets ex¬ 
tend about 50 miles. 



Fig. 114. Flying conditions common in mT air of the 
subtropical Pacific 















20,000 


15,000 


10,000 


5,000 


Fig. 115. Flying conditions common in polar air 
of the subtropical Pacific 

Flying Conditions in Polar Air of the Sub¬ 
tropical Pacific: 

Sky .Four- to five-tenths cloud cover. 

Cloud types .Dominantly cumulus (hard look¬ 

ing, sharply defined). 

Ceilings.2500 feet average. 

Cloud tops .4000 feet average. 

Precipitation.Instability showers. 

Turbulence.None in free air. 

Slight below cloud bases. 
Moderate in clouds. 

Visibility.Hazy below 4000 feet (hazier 

than in mT air). 

Icing level.14,000 feet average. 

Winds .Surface winds are northeasterly, 

northerly or northwesterly. 
Winds aloft (above 10,000 feet) 
are westerly. 

Special features ....Frontal cyclones and migratory 
highs. 


-0° C - 

No icing danger below 14,000ft. 

Hazy below 4,000 ft. 


Flying Conditions in Cold Fronts of the Sub¬ 
tropical Pacific: 


Width of 

“weather belt”.50 to 100 miles. 

Direction of 

movement.Southeastward. 

Weather .Squalls and thunderstorms. 

Cloud types .Cumulo-nimbus and heavy cum¬ 

ulus, with alto-cumulus ahead of 
the front. 

Ceilings .1000 feet average. 

Cloud tops ....12,000 feet average. 

Turbulence.Moderate in clouds below 10,000 

feet. 

Severe in clouds above 14,000 
feet. 

Icing .Above 10,000 feet average. 


(Best flight altitude in typical cold front, such as 
that outlined here, is 10,000 feet. Why?) 

Flying Conditions in Warm Fronts of the Sub¬ 
tropical Pacific: 


Width of 

“weather belt”.100 to 200 miles. 

Direction of 

movement.Eastward. 

Weather .Rain or drizzle. 

Cloud types .Cirro-stratus and nimbo-stratus. 

Ceilings.600 feet average. 

Cloud tops .Nimbo-stratus, 8000 feet average; 

alto-stratus, 12,000 feet average. 

Turbulence.Slight. 

Icing .Above 10,000 feet average. 


(Best flight altitude in typical warm front, such 
as that outlined here, is about 8000 feet. Why?) 


20 , 000 - 

15,000- 
10 , 000 - 

5,000- 



SCALE 


X 



£ 

25m/. 

Hor/zonfa/ 


Icing danger in cloud above 10,000 ft. 



€ 3 - 


Polar air 


Best Flight T rock 

A pV' 

J hazy below ( J 
flOOO ft. 

mT air 


Id 


- 20 , 000 ' 

■ -15,000' 

- 10 , 000 ' 

-5.000' 


300 mi. 

Fig. 116. Flying conditions in a typical winter cold front of the subtropical Pacific 


117 
















































WEATHER MAPS AND FLIGHTS OF 
THE NORTH PACIFIC 

Map Series Illustrating Cyclonic Sequence 

Figures 118 to 123 are weather maps for December 
17 to 22, 1938, which illustrate the characteristic 
frontal movement over the wide expanse of the 
North Pacific Ocean. You know, of course, that 
identical situations rarely occur, and no series of 
maps can be called typical. Yet the mechanics of 
cyclone development and dissipation does follow 
a fairly well established and proven pattern. This 
series of maps furnishes an example. 

On December 17th (Figure 118), the predominant 
feature of the weather map of the Pacific Ocean is 
the deep low (975 mb.) centered over southwestern 
Alaska from which a long occluded front extends 
southward to the eastern part of the ocean. Farther 
west is another cyclone in the process of occlusion. 
These, together with the occluded front east of 
Japan, constitute a family of cyclones on the princi¬ 
pal polar front of the Pacific. The storm off the coast 
of southern California is a cyclone on the secondary 
polar front. 

We shall follow the development of these cy¬ 
clones, especially the front east of Japan which we 
will refer to as “Bertha”. 

On December 18th (Figure 119), the cyclone 
family has moved east. The one from southwest 
Alaska has occluded out over western Canada, and 
its position in the eastern Pacific has been taken by 
the second cyclone. The cold front extension of this 
cyclone makes a very shallow trough between the 
two high pressure cells to the south. 

118 


Bertha has had a very active time during the 24- 
hour interim. A wave has formed on the front and 
has become partially occluded as this entire system 
moved eastward. The little storm in the southeast is 
now occluding and moving close to California. 

On December 19th (Figure 120), the front that 
held the center of the stage on the previous day is 
now completely occluded, and Bertha is moving into 
Alaska with a family of three following her along 
the principal polar front. The California storm has 
moved inland and has become pretty well occluded. 

On December 20th (Figure 121) Bertha has 
occluded along the Alaska coast, and her family, 
growing up (in various stages of occlusion), dom¬ 
inates the entire frontal situation of the North 
Pacific. 

The California storm has occluded to a cold front 
aloft, and an outbreak of cP air from the continent 
forms a cold front over northern California* and a 
stationary front along the Pacific coast of North 
America. 

On December 21st (Figure 122) Bertha has oc¬ 
cluded to a cold front aloft, her eldest has attached 
itself to the stationary front along the coast, while 
her youngest extends a long cold front southward 
between two high pressure cells of the subtropical 
high. This industrious child bids fair to becoming a 
major secondary polar front in the mid-Pacific. 

On December 22nd (Figure 123) “baby dood it”! 
A secondary polar front is established and we may 
expect that storms will begin to affect the west 
coast farther to the south. Also, a new family can 
be expected to be developing in the northwestern 
Pacific, not shown on this map. 






























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119 


118. Weather map, December 17, 1938 





































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120 


119. Weather map. December 18, 1938 






































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121 


120. Weather map, December 



















































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122 


121. Weather map, December 20, 1938 































123 


122 Weather map. December 21. 1938 



































124 


123- Weather map, December 22, 1938 


































bp 


125 


124. Weather map, October 11, 1942 


























bh 
• « 


126 


125. Weather map, October 12, 1942 



































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127 


. 126. Weather map, October 13, 1942 
























Weather During Individual Flights 

Figures 124 to 126 show the weather over the 
eastern Pacific Ocean on October n, 12, and 13, 
1942, and the reports of a flight from Honolulu to 
San Francisco, and from San Francisco to Honolulu. 
These are “post-flight” reports and were added to 
the weather maps after the flights were completed. 

The weather map for October nth was drawn 
at 1830 G.C.T., and shows the principal formation 
of the area to be a low (996 mb.) centered over 
southwestern Alaska. From this low extends a simple 
warm and cold front which has partly occluded, 
the cold front extending southwestward between 
the two semipermanent high pressure cells. To the 
east of this formation lies another and weaker low, 
with the southern extension of the cold front making 
a slight trough through the center of the high. Off 
the coast of California is a cold front of moderate 
intensity which is a part of a cyclone family extend¬ 
ing from the tropics to the Arctic regions and bring¬ 
ing some bad weather to the west coast of the 
United States. 

A cross section of the flight from Honolulu to 
San Francisco (Figure 124) shows how the flight 
was made and the weather as encountered. (The 
take-off and landing times are not given.) While fly¬ 
ing in the subtropical high, no weather of conse¬ 
quence was encountered other than some high clouds 
near the shallow cold front which penetrated the 
high. However, at 1200 G.C.T., the crew discov¬ 


ered that they were bucking headwinds and dropped 
down to 4000 feet, at which level they made their 
1300 report. At 1400, they reported that they were 
again flying at 9000 feet. As they approached the 
cold front off the California coast, they let down to 
2000 feet in order to fly under the scattered frontal 
thunderstorms. As they approached the coast, the 
thunderstorms became more numerous and further 
descent to 1000 feet (1900 G.C.T.) was made. 

On October 12th (Figure 125) the low centered 
over southwestern Alaska on the preceding day has 
moved northeastward and is now centered (987 mb.) 
over the western part of the Gulf of Alaska. Along 
the cold front of this disturbance a wave has also 
formed. Other formations of the previous day have 
moved eastward and are either off this map or of no 
further consequence in this area. 

On October 13th (Figure 126) the center (972 
mb.) of the low in the western Gulf of Alaska has 
remained almost stationary, but the occluded front 
has moved eastward and lies along the northwest 
coast of the continent. The secondary wave has 
advanced westward and increased in intensity. 

The flight reports show excellent flying conditions, 
as is characteristic in the subtropical high. Layers of 
broken strato-cumulus clouds at 5000 feet, and a 
steady headwind to the center of the high, and then 
steady tailwinds, were reported. In fact, the weather 
was so routine that a complete report for the trip 
was not recorded on the map. 


128 


C^S- CENTRAL AND SOUTH PACIFIC -S^O 


Are you “taken” by romantic-sounding names of foreign places? Do Pago Pago, Honolulu, Tonga 
Fiji, stir your blood? Do you think of Dorothy Lamour in a sarong, and of the hula hula? This region 
is (or was) a land of fair weather and romantic fancies. Though the romantic bubble has burst, the 
region remains a land of fair weather. Lying south of the stormy Pacific described in Chapter 5 and east 
of the monsoon belt, to be treated in Chapter 7, this area is one of tropical weather par excellence 
(Figure 127). 


You have learned about the tropics in Chapter 4, 
but the tropical Atlantic, especially in its West 
Indian section, is disturbed by northers (or cold 
surges) from the North American continent. To 
an even greater extent the monsoon belt of the 
Southwest Pacific feels the continental influence— 
it feels it in the form of the Asiatic monsoon. In 
this central portion of earth’s largest ocean, however, 
continental influence is almost entirely lacking. The 
climate of the Central Pacific (Latitude 30°N. to 
3o°S. and east of the 145°E. Longitude) almost fits 
the ideal climatic pattern—the climatic pattern that 
one would expect on an earth of uniform composi¬ 
tion. Only small islands complicate the picture. 

Even “ideal” tropical weather, however, has a 
few intricacies and hazards. You have learned of 
the intertropical front and of tropical cyclones. We 
will need to find out where and how these disturb¬ 
ances operate in the Central Pacific. 

Also in this chapter we will hold an aerological 
“Neptune’s Court,” initiating you into the compli¬ 
cations that occur when an air mass crosses the 
equator. After this you will be a “full-fledged salt,” 
having been introduced to Southern Hemisphere 
weather, which pilots call “upside-down” weather. 
It’s also sometimes called “mirrored” weather, for a 
reason that you’ll soon know. You understand that 
wind differences in the two hemispheres are caused 
by the force (deflecting force, or Coriolis force) 
produced by earth’s rotation—a force that deflects 
winds to the right in the Northern Hemisphere and 
to the left in the Southern Hemisphere. The reversal 
of deflection, as you cross the equator, will play a 
lot of tricks on you unless you use your head. 


THE WIND PATTERN 

Since the Pacific is a large homogeneous (all 
ocean) area, the general air circulation of the Cen¬ 
tral Pacific should be expected to approach the 
hypothetic ideal pattern (Figure 7). It does. A 
thermally produced low-pressure belt lies near the 
equator, and is flanked in both hemispheres by sub¬ 
tropical highs. From subtropical highs to the Equa¬ 
torial low flow the trade winds—NE. trades in 
the Northern Hemisphere and SE. trades in the 
Southern. As the trade wind air moves toward the 
equator it warms and expands. Converging near the 
equator, the expanding air in the two trade wind 
currents rises in doldrum belt or intertropical front 
activity. 

The intertropical front, or zone of convergence 
between the two trade wind systems, migrates with 
the heat equator. It moves a few degrees north for 
the northern summer, a few degrees south for the 
southern summer. Note on Figure 128 its seasonal 
locations through the Central Pacific. A SE. trade 
that must cross the equator to reach the intertropical 
front becomes a southwest wind north of the equator. 
Likewise, a NE. trade that continues across the 
equator becomes a northwest wind in the Southern 
Hemisphere. Aloft, the air that ascends in the equa¬ 
torial belt returns to the subtropical highs (or moves 
even farther poleward to settle in polar high-pressure 
areas). Moving away from the equatorial belt, this 
upper air feels the influence of the deflecting force. 
Deflecting to the right in the Northern Hemisphere, 
it becomes a WSW. wind. Deflecting to the left in 
the Southern Hemisphere, it becomes a WNW. wind. 
In each hemisphere the upper air thus moves from 
a direction roughly opposite that of the trade wind 


129 



130 















































































Fig. 128. Position of the intertropical front, February-March and August-September 


beneath it and is often called the “antitrade” (see 
Figures 7 and 129). 

At its source, near the center of a subtropical high, 
a trade wind current is only about 8000 feet deep. 
At 20 0 Latitude, it reaches a depth of 10,000 feet. 
It continues increasing in thickness until, near the 
equator, it has become a deep current that extends 
to 30,000 feet. Along the upper limit of the trades a 
zone of variable winds inclines from about 10,000 
feet at 20 0 Latitude to about 30,000 feet at the equa¬ 
tor. Above this zone the westerly “antitrades” pre¬ 
vail. Figure 129 shows a generalized cross section of 
this tropical wind system. 

Because of the seasonal shift or migration of the 
entire tropical wind system, the thickness of the 
trade wind and the altitude of the westerly antitrade 
at any latiti^de varies with the season. During winter 
of the hemisphere concerned, the trade current is most 
shallow and the westerly wind aloft is encounted 
at lowest elevations. 

The surface winds of the trade wind belt are 
steady over 80% of the time and are strongest in 
winter, when they average 10 to 16 knots. In general, 
velocities increase from the surface to 3000 feet and 
diminish above 8000 feet, where a gradual transition 


to the antitrades begins to take place. Refer to 
Figures 130 and 131. 

WEATHER IN THE DIFFERING AIR MASSES 
AND FRONTS OVER 
THE CENTRAL AND SOUTH PACIFIC 

Air Mass Weather 

Tropical Maritime Cold (mTk). Over most of 
the Central Pacific the prevailing air mass is mTk 
or trade air. In the absence of orographic influences, 
as over the open ocean, the weather is fine and clear, 
for the air moves from warm to only slightly warmer 
seas. With increased distance from its source the air 
becomes increasingly humid (specific humidity in¬ 
creasing, but relative humidity remaining steady), 
and warming of the lower layers of the air results 
in some instability. Yet the sea over which it travels 
is so nearly the same temperature all the way that 
the characteristics of the air mass change only slightly 
in its travels. It remains reasonably stable, though 
warm and moist. In this air at latitudes near or north 
of 20 0 , a temperature inversion is evident at about 
8000 feet. Below the inversion level the air is hazy, 
reducing visibility somewhat; but above the inver- 



30,000* 


— 20 , 000 * 


10 , 000 * 


30° N.Lat. 
(Near center of 
Subtropical hiqh) 


20 N.Lat. 10 N.Lat. Equator 10°3.Lat. 20\S.Lat. 30°S.Lat. 

igggg VARIABLE WIND5 ( Near center of - 

Fig. 129. A generalized cross section of the tropical wind system 


subtropical hiqh) 


132 


































































133 


Fig. 130 Fig. 131 





















































































































sion, visibility is good. Frequent patches of ragged- 
looking cumulus clouds develop, but air mass thun¬ 
derstorms hardly ever occur. Cloud bases under the 
scattered cumuli are usually about 2000 feet and 
the cumulus tops extend to about 8000 feet (Figure 
132). Because of the warm temperature of tropical 
oceans, the icing level is usually above 15,000 feet, 
and throughout the year there is little if any danger 
of icing below 10,000 feet. You’re safe from icing 
except in the rare cumulus clouds that reach to 
13,000-18,000 feet. See p. 116 for outline and dia¬ 
gram of flying conditions in mT air. 

Equatorial (E). As the mT air approaches the 
equator it gradually changes into equatorial air with 
high temperature and high moisture content at all 
levels. 

The high temperature of 8o°F. to 84°F. (see Fig¬ 
ures 13 3 and 134) and abundant moisture are favorable 
to vigorous convection with copious rainfall (yearly 
average 60 to 100 inches).* Winds are light and 
variable, and calms are frequent. You’ll find more 
and more thunderstorms as you fly toward the 
equator, especially in the vicinity of island groups. 
Individual storms are usually small, but they may 
develop to as much as 100 miles in diameter. 

The cloud types in equatorial air are heavy 
cumulus and cumulo-nimbus with alto-stratus at high 
levels. Ceilings average 400 to 1000 feet, and cloud 
tops extend to 16,000 or even to 30,000 or 40,000 
feet. Best flying conditions over equatorial seas, if 
cloudy, are between 300 and 1000 feet. 

Frontal Weather 

The Intertropical Front. As the trades converge 
near the heat equator they form a front or frontal 
zone which extends east-west indefinitely. As a zone 
it forms the doldrum belt, but when the edges of 
the zone converge a single' front is formed. The 
single front has well-defined discontinuities, princi¬ 
pally in wind direction and velocity, and sometimes 
in humidity and temperature. This front may be 
either of cold front or warm front type. It is 
normally most active when located farthest from 
the equator, as during the summer season of one of 
the hemispheres. During an Equinox season there is 
usually a wide and pronounced doldrum belt. In the 
absence of this belt, there may be a simple easterly 
flow of the two trade winds along the equator, with 
only slight mixing at their boundary. This will pro¬ 
duce a certain amount of cloud and showers but no 
intense activity. 

* Fronts to be described later, account for part of this heavy 
rainfall. 



Fig. 132. Marcus Island, with fair weather cumulus and smoke 
from Navy raid (U.S. Navy photo) 


The intertropical front follows the heat equator, 
migrating seasonally with changes in the sun’s de¬ 
clination (Figure 128). Owing to the unequal heating 
of land and water, and to the transfer of heat and 
cold by air and water currents, the front shifts dif¬ 
ferentially and becomes deformed in shape. Thus, 
the strong sweep of the cold water of the Humbolt 
(or Peru) Current so materially lowers the tempera¬ 
ture of equatorial waters in the eastern Pacific that 
the intertropical front remains in the Northern Hemi¬ 
sphere throughout the year (Figure 128). 

Reaching its most southerly position in February-. 
March, the front lies along the 5th parallel north 
over the ocean east of i5o°W. Crossing the equator 
at 150°, it extends across the Samoa, Fiji and New 
Hebrides Islands and fans out into a broad belt over 
New Guinea and northern Australia. 

The shift of the front is not steady, but is accom¬ 
plished through a series of day-to-day and week-to- 
week advances and retreats. During retreat of the 
converging trade on one or both sides, a doldrum 
area forms or broadens. A renewed surge causes 
the doldrum area to narrow and frequently causes 
a single advancing front. The stronger surges usually 
come from the hemisphere which is in winter season. 
In this way the belt is shifted to the warmer 
hemisphere. 

By August it has reached a position about io°N. 
in the eastern Pacific, extending across the Marshall 
and Caroline Islands and fanning out north of the 
Philippines to Taiwan and South China (Figure 128). 
In this position there is a well-defined discontinuity 
in wind direction along the entire front. The 


134 





135 


Fig. 133 Fig. 134 
















































































































































Southern Hemisphere trades have crossed the 
equator and, becoming subjected to right-hand de¬ 
flection, are diverted from their southeast direction 
to become south and southwest winds. There is thus 
a meeting along the front of southwest and northeast 
winds. 

The southwest winds, having come from the 
winter hemisphere, are slightly the colder; there¬ 
fore, when their velocity is the higher, a northward 
advancing cold front results. The frontal slope is 
very low, less than i in 400 (1 mile altitude to 
400 miles horizontal) up to about one mile eleva¬ 
tion, where it becomes almost parallel to the surface. 
The passage of this front over a station is accom¬ 
panied by thunderstorms and heavy rain, but after 
about an hour the heavy rain “lets up”, the heavy 
cloud is replaced by alto-stratus, and light rain or 
drizzle falls for 2 or 3 hours. The width of the 
rain area is usually about 200 miles, but it may be 
as great as 400 or as small as 50 miles. 

When there is an increase in the NE. trades, the 
front will retreat as a warm front and light rain 
from alto-stratus may fall for 1 or 2 days. 

During the Southern Hemisphere summer, a simi¬ 
lar action takes place south of the equator (west of 
longitude i5o°W.) between the SE. trades and the 
northwest winds of the diverted NE. trades. 

The Doldrums. “Doldrums” is a name applied by 
the mariners of old to the customary belt of calms 
or light variable winds which usually lies along the 
heat equator. When caught in this stagnant air 
with no wind to fill their sails, the masted schooners 
would loll in the blistering tropical sun for days. 
This inactivity depressed the spirits of the sailors, 
making them listless, dull, and “in the dumps”. In 
other words, they were in the “doldrums”. 

They were SAILors in the literal sense, but you 
are an aeronaut and your experiences will be quite 
different from those whose bottoms floated on 
the glassy sea. 

Actually, the doldrum belt appears as a series of 
“loops” in the general intertropical front (Figure 
135). There is little frontal activity along the edges 
of these “dead spots” except when the borders are 



Fig. 135. Doldrum belt as loops on the intertropical front 


136 



Fig. 136. Doldrum belt with warm front at each edge 

closing in and the zone is disappearing. The transi¬ 
tion from frontal to doldrum conditions is very rapid 
and is caused by a dropping off of the wind on 
one or both sides of the front. At times a very active 
front may be transformed into a wide doldrum belt 
within 24 hours. 

At an edge of a doldrum belt, a trade air mass 
may form either a cold front or a warm front. Thus, 
the doldrums are of three types as shown in Figures 
136, 137, and 138. (Note: The direction of either 
trade wind may be diverted if the front is located 
in the opposite hemisphere.) 

In Figure 136 the doldrum area is bounded on both 
sides by more or less steady streams of air which 
follow the boundaries of the area in ENE. and ESE. 
directions respectively. If this illustration were not 
centered on the equator one of the trade winds 
would be diverted. The streams flow over the dol¬ 
drum area and converge into one steady easterly 
stream above z, leaving a wedge (xyz) of stagnant air. 

Ever since you first heard of flying, you have 
heard of “air pockets”. According to popular belief 
an air pocket is a place where there is no air to sup¬ 
port a plane, so it drops. Of course, no such thing 
as a vacuum can exist in the air. Here, however, is 
an “air pocket”, but remember there is air in the 
“pocket”. It’s stagnant air—light and expanded and 
not flowing. 



















This type of doldrum is very persistent. The 
bordering winds do not move the edges very far, 
and the condition may remain unchanged for days. 
Weather in this type of doldrum is usually fair, with 
little cloudiness unless the overriding winds are quite 
unstable. 

In Figure 137 the boundary winds undercut the 
stagnant air forming cold fronts at both sides. 
Any slight change in wind velocity will produce 
thunderstorms (or at least large cumulus clouds) at 
the edges of the belt. At the center of the area there 
is little if any cloudiness. The edges of this system 
fluctuate considerably in position, and with each 
new surge of air thunderstorms or line squalls occur. 

In Figure 138 the air from one hemisphere under¬ 
cuts the stagnant air on one doldrum border, while 
the air from the opposite hemisphere overrides the 
stagnant air. This is a fairly frequent type of doldrum 
belt. The undercut (cold front) side generally fluc¬ 
tuates, while the warm front side is fairly persistent 
in position. As the cold front moves in, destroying 
the doldrum area, the two trades meet. This develops 
a single front. 

The Subsidiary Front. A subsidiary front, often 
developing great activity, may form a “triple point” 
with the intertropical front, when it is beyond the 
equator in the hemisphere having summer. Figure 
139 illustrates the formation of this type of front 
in the Northern Hemisphere. The explanation is that 
when an air mass crosses the equator, part of it will 



Fig. 137. Doldrum belt with cold front at each edge 


continue in its path undiverted and part will change 
into a southerly or southwesterly stream.* 

A subsidiary front is frequently formed between 
these portions of the same air mass. This front nor¬ 
mally reaches only about 5000 feet in height, and 

* The difference in degree of diversion of the separate por¬ 
tions of the air mass seems to depend upon the section of the 
anticyclone from which each part was derived. 



137 
























































Fig. 139. Subsidiary front 

may have a steady stream of air flowing above it. The 
air masses of the trades are usually conditionally un¬ 
stable; therefore, when one portion of the air mass 
meets the other with even a slight discontinuity of 
wind direction, sufficient lifting results to upset the 
balance and very severe weather may result. 

General Flying Weather Through Doldrum 
Areas and Intertropical Fronts. Frontal activity 
of equatorial areas normally increases at night and 
reaches a maximum in the early morning. During the 
day, a front often loses most of its activity although 
the sky may remain overcast. 

Flying conditions in the intertropical front and 
doldrum zone may be summarized as follows: 


Weather .Intermittent rain, squalls and 

thunderstorms. 

Cloud .Much heavy cumlus and cumulo¬ 

nimbus, with attendant stratus of 
all levels. 

Cloud Tops .16,000 to 25,000 feet average. 

Ceiling.1000 feet. 

Icing .None below 15,000 feet. 


Best flight altitude—800 feet. 

Polar Front. Polar front activity affects the 
weather along the poleward borders of the area 
under discussion in both hemispheres. The circula¬ 
tion along the northern border was described in 
Chapter 5 and need not be repeated here. Note that 
the Hawaiian Islands, and especially Midway, are 
subject in winter to occasional polar front squalls 
known as “Kona storms”. (See p. 119 for outline 
and diagram of flying conditions in polar fronts of 
the subtropical North Pacific.) 

Fronts of the South Pacific. It is very difficult 
to obtain an adequate analysis of the weather of 
the Southern Hemisphere because of the great pre¬ 
dominance of water. Not only are there few estab¬ 

138 


lished weather stations but the ocean is little traveled, 
which results in few ship reports. Nevertheless, 
sufficient data has been obtained to establish the fact 
that frontal activity, similar to that of the Northern 
Hemisphere, does exist. Investigation has not, how¬ 
ever, progressed to the point of permitting a thor¬ 
ough understanding of the movements of the fronts. 

Figure 140 shows the contrast in the cyclone dia¬ 
gram north and south of the equator. The Southern 
Hemisphere diagram is a mirror-image of the dia¬ 
gram for the Northern Hemisphere. Thus, in the 
Southern Hemisphere, the warm sector is the north¬ 
east instead of the southeast sector. Warm winds 
blow from the northwest; whereas cold winds, rush¬ 
ing in behind the cold front, have a southerly or 
southeasterly direction. 




Fig. 140. The extratropical cyclone of the Southern Hemisphere 
compared to the extratropical cyclone of the 
Northern Hemisphere 
































Many of the extratropical cyclones of the South¬ 
ern Hemisphere occur south of the area in which 
we are interested, but occasional outbursts of polar 
air develop fronts and cyclones in southern Australia 
and New Zealand. The series of maps, February 2 
to 4, 1941, shows a typical situation in this area 
(Figures 141, 142 and 143). 

Normally the subtropical high extends across the 
South Pacific between latitudes 25 °S. and 40 °S. 
When, however, a well-developed front forms in the 
belt of “Roaring Forties”, the northern end of the 
cold front extends into the subtropical high, dividing 
it into separate cells of high pressure. The cold front 
moves eastward. 

Figure 141 shows this situation. A cold front, 
without the customary warm front, appears in the 
trough between two high pressure cells. On the 
following days (Figures 142 and 143), waves de¬ 
velop along the front as it crosses the Tasman Sea 
and the occlusion occurs over New Zealand. 

The weather maps for October 24 and 25, 1940 
(Figures 144 and 145), show another development 
of weather along a trough between the anticyclonic 
cells. On these maps, post-flight reports are shown 
together with cross sections of the flights. On 


October 24th, a pilot found considerable cloudiness 
and showers on both sides of the front over the open 
sea. Crossing the front at about 2300 G.C.T., the 
pilot encountered rain, low visibility and low ceil¬ 
ings. On the next day the line of flight was some¬ 
what parallel to the front and 200 to 300 miles 
behind it. Along this flight path, cloudiness pre¬ 
vailed in the form of stratus and strato-cumulus with 
high clouds of cirrus and alto-cumulus, necessitating 
flight at an altitude of approximately 10,000 feet. 
Comparison of the two maps shows that the warm 



THIS IS HOT WHAT IS MEANT BY "UPSIDE-DOWN WEATHER" 


139 























Fig. 142. Weather map, February 3, 1941 



140 




















































front sector of October 24th became a stationary 
front by the 25th, and the cold front continued 
eastward. 

Pilots should note that, like the polar fronts and 
I extratropical cyclones of the Northern Hemisphere, 
Southern Hemisphere polar front formations move 
j from west to east. In all other respects the forma¬ 
tions are “upside-down”. For example, when a pilot 
encounters a front in the middle latitudes of the 
1 Northern Hemisphere he generally can find better 
flying weather by turning south , for the center of 
| the storm is usually to the north. In the Southern 



Hemisphere, however, it is usually necessary to fly 
north to avoid centers of depressions. (See Figure 
140 and the Southern Hemisphere weather maps.) 
Altogether too frequently pilots new to Southern 
Hemisphere flying have thoughtlessly flown south 
to get around a front, only to recall (sometimes after 
an hour or more) that this was flying toward, not 
away from, the storm center. In several instances 
the loss of time and fuel, due to such miscalculation, 
j has prevented the pilot from carrying out his mission. 

It must also be remembered that the wind directions 
around high and low pressure areas are the reverse 
of the wind circulations in the Northern Hemisphere. 




To find the centers of cyclones and anticyclones 
in the Southern Hemisphere the law is: If you turn 
your back to the wind, the low pressure is to your 
right and the high is to your left. 



WHEN YOU STRIKE A SOUTHERN HEMISPHERE COLDFRONT IN 
THIS POSITION — TURN NORTH 


141 







142 


Fig. 144. Weather map, October 24, 1940 
































143 


Fig. 145. Weather map, October 25, 1940 















































TYPHOONS OF THE TROPICAL NORTH PACIFIC 


Month 

Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

Year 

Average 

Frequency 

1.2 

0.7 

0.7 

0.6 

1.3 

1.4 

3.6 

3.7 

4.9 

3.9 

2.1 

14 

,5, 


This table includes all typhoons from i6o°E. Longitude to the coast of Asia. 


Tropical Cyclones 

The characteristics of tropical cyclones were de¬ 
scribed in Chapter 4. You know the observable 
weather signs that warn of the approach of one 
of these storms and indicate its direction from you, 
and you know something of the structure of the 
storm, with its “dangerous semicircle” and its (only 
slightly less dangerous) “navigable semicircle”. This 
information may help save your skin by enabling you 
to steer clear of these intemperate terrors. 

Note now, relative to the Central Pacific, where 
these storms are likely to be encountered and at 
what season of the year they are most frequent in 
each region. Two regions of the Central Pacific have 
tropical cyclones (see Figure 146). North of the 
equator and westward from i6o°E., they may be 
encountered between 5°N. and i5°N. on any month 
of the year. Here they are called typhoons. The 
worst typhoon season is from July to November. 
The frequency by months is shown in the table above. 

These storms originate in the Caroline and Mari¬ 
anas Islands and intensify as they move westward 
toward the Philippines. Their speed of advance 
varies. Generally they move at a rate of 6 to 9 knots 
while going westward, and speed up to 15 or 20 
knots after recurving to the north and northeast. 
A study of Figure 146 shows that the dangerous 
semicircle of many typhoons crosses the western 
border of the area under discussion, i.e., from the 
Caroline Islands northward to Guam and the Bonin 
Islands. 

Typhoons are rare (or absent) on the air route 
from Hawaii to the Phoenix Islands. To the east of the 
area under discussion, along the west coast of Central 
America and Mexico, tropical cyclones called cor- 
donazos occur from August to November. An occa¬ 
sional storm from near these American shores, failing 
to recurve, may move westward and die out in the 
vicinity of the Hawaiian Islands. 


No tropical cyclones have ever been reported 
between 5°N. and 5°S., but the outer fringes of 
storms at higher latitudes may cause difficult weather 
in this area. 

Most of the tropical cyclones affecting the 
Southern Hemisphere section of the Central Pacific 
originate between 5°S. and i5°S., and between 
i6o°W. and i6o°E. They move westward a short 
distance and recurve to the southwest, south, and 
finally southeast. Over these Southern Hemisphere 
seas the tropical cyclone season, as shown below, is 
November to April. 

The attention of flight crews should be called to 
the following special features of tropical cyclones in 
the Southern Hemisphere: 

(1) Circulation around tropical cyclones in the 
Southern Hemisphere is clockwise. Thus, when you 
turn your back to the wind, the center of the storm 
is to your right. 

(2 ) A storm center moving westward may recurve 
in any direction between 130° and 270°, thus in the 
Southern Hemisphere the left half of the storm is 
the “dangerous semicircle”. 

It is important for pilots flying in any area of 
tropical cyclone activity to regard as suspicious all 
of the following observations: 

(1) Heavy cloud too high to climb over. 

(2) Increasing wind velocity. 

(3) Changing wind direction. 

(4) Heavy swells (especially if not with the wind). 

(5) Lightning and squalls. 

(6) Increased turbulence. 

(7) Falling altimeter reading (if close enough to 
water to maintain constant altitude). 

(9) Peculiar cloud forms (distortion caused by 
high winds). 


TROPICAL CYCLONES OF THE TROPICAL SOUTH PACIFIC 


Month 

Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

Year 

Average 

Frequency 

2.0 

1.5 

1.9 

0.5 

0.1 

0 

0 

0 

0.1 

0.1 

0.2 

1.0 

7.4 


144 






































Fig. 146. Typical tropical cyclone tracks in Central 
and South Pacific 


FLYING WEATHER OF SPECIAL AREAS 
Pearl Harbor—Midway—Wake—Guam 

The major pre-war air route across the Pacific 
skirted along the northern border of the area dis¬ 
cussed in this chapter. Island bases were a determin¬ 
ing factor in the location of this route, but fortu¬ 
nately the area has the added advantage of much 
good flying weather. 

Hawaii and Midway have some polar front weather 
in winter. Guam, on the western border of the 
Central Pacific, has some intertropical front weather 
in summer (Figure 128). The air route from Hawaii 
to Guam, therefore, extends from an area of winter 
polar front storms with fair trade wind summer 
weather to an area of summer intertropical front 
activity with fair trade wind winter weather. 

The invasions of polar air that reach the Hawaiian- 
Midway area during the winter season produce rela¬ 
tively mild frontal activity (owing to modification 
of the polar air before it gets this far from its home 
port). These are the weak equatorward ends of 
North-Pacific polar fronts, but they can cause dis¬ 
turbing flying difficulties, especially near Midway. 
From October to March the polar front with its 
extratropical depressions affects Hawaii and Mid¬ 
way, and during February (the height of the North 
Pacific winter) occasional polar outbursts reach 
nearly to Wake. The worst weather is encountered 
in the region of Midway, where cold fronts cause 
squalls and winds to 65 knots or higher. Rain falls 
at Midway on more than half the days during 
December, January and February. In extratropical 
disturbances, visibility lowers and ceilings drop to 
below 1000 feet. Light to moderate icing may be 
expected in the frontal clouds above 10,000 feet. 


Where the disturbances pass over high mountains of 
the Hawaiian Islands, the rate of icing increases 
rapidly, and severe icing may be expected where 
fronts hang (retarded) above the mountains. 

During most of the year the area enjoys trade 
wind weather—clear and fine over the oceans and on 
leeward coasts, but rainy on windward mountain 
slopes. So-called “thick” weather (to the extent of 
interfering seriously with flying) is almost unknown 
and is usually confined to mist or rain. Fogs seldom 
form. Windward and leeward slopes have pro¬ 
nounced differences in rainfall. The world’s record 
occurs in central Kauai, in the Hawaiians. There, 
near the summit of Mount Waialeale, at an elevation 
of 5075 feet, the average annual rainfall is over 450 
inches; while 15 miles to the southwest, on the lee¬ 
ward side, it is less than 20 inches. In the Territory 
of Hawaii as a whole, considerably more rain falls 
in winter than in summer. 

in Wake and Guam summer is the stormier period. 
During July, August and September the intertropical 
front and tropical cyclones affect the region west of 
Wake. Airline pilots flying the Wake-Guam-Manila 
track reported crossing the intertropical front on 
every summer (July-August-September) flight 
through this region. They found good trade weather 
on the eastern side of the front and squally monsoon 
weather on the western side. 



- HAWAIIAN ISLANDS - 

LEEWARD SIDES WINDWARD SIDES 

OF MOUNTAINS OF MOUNTAINS 

ARE DRY ARE RAINY 


145 













Typhoons that move northwestward near Guam, 
or recurve north and northeast, cause cyclonic winds 
of high velocity. From July through November, 
typhoons affect Guam, but Wake and the Hawaiian 
Islands have never experienced them. 

Line, Marshall and Caroline Islands 

These groups of islands lie between i5°N. and the 
equator. The Marshalls and Carolines were Japanese 
mandate islands, the Line Islands belonging to the 
United States and Great Britain. This of course 
meant that the Marshalls and Carolines were in for 
severe air attacks. Climatically, also, the ex-axis islands 
are the more afflicted. Typhoons sweep the Carolines, 
while none develop farther east (Figure 146); and 
also more severe rain squalls occur in the Marshalls 
and Carolines than in the Line Islands. 

Both the squalliness of the Marshalls and Carolines 
and the typhoons of the Carolines occur when the 
intertropical front and doldrum activity lies across 
these Japanese islands. This is in the northern sum¬ 
mer, from May to November (Figure 146). The 
activity in the fluctuating intertropical frontal zone 
and the many air mass thunderstorms in the warm 
equatorial air make this the rainy season—and the 
season of most difficult flying—in the Marshalls and 
Carolines. The air mass thunderstorms are usually less 
than 25 miles in diameter (never more than 100 
miles) and may be avoided. In frontal squalls, ceil¬ 
ings may lower to below 1000 feet and the generally 
excellent visibility may be reduced by rain. 

“Winters” are different. From December through 
April the intertropical front is south of the equator, 
permitting the NE. trades to sweep over the Mar¬ 
shalls and Carolines. During this season the ocean 
areas have fair weather with only scattered cumulus 
clouds. Showers occur over the islands on about one- 
half the days, and windward slopes have much 
cloudiness; but your principal flying hazard will be 
“ack-ack”. 

Over the Line Islands the intertropical front is at 
its weakest. Here the trades from the two hemi¬ 
spheres converge to form easterly winds that usually 
flow side by side with little frontal activity between 
them. Except on windward slopes, it rarely rains 
with east winds. Sometimes, however, a surge of air 
from the northeast undercuts the easterly winds from 
the Southern Hemisphere and produces rain with a 
northeast wind. 

As elsewhere in the tropics, the intertropical front 
of the Line Island area migrates with the heat equa¬ 
tor. It lies near the equator (south of the Line Is¬ 
lands) in January, and at about 8°N. Latitude (over 
the northern islands “of the Line”) in July. This 

146 


causes a certain seasonality in precipitation, with 
most of the islands having two rainy periods each 
year—rainy weather when the frontal zone moves 
over going north, and rainy weather when it riioves 
over going south. 

In this tropical area you may encounter 'water¬ 
spouts. Some of these are the tropical maritime 
cousins of the American tornadoes that harry our 
Great Plains and Gulf Coast. When a “chimney” of 
violent uprush develops over the sea, setting in mo¬ 
tion a huge and violent whirlwind, the extremely 
low pressure in the vortex of the whirl sucks sea 
water upward into the funnel-shaped tornado cloud. 
A waterspout of this type can exert all the destruc¬ 
tive force of a tornado on land. 

Many waterspouts, however, have only whirlwind 
rather than tornado proportions and intensity. In 
these less destructive “spouts”, the funnel cloud 
never contains sea water; instead it is merely a whirl¬ 
wind column of cloud that results from the adiabatic 
cooling of the rotating, ascending and expanding 
humid air. Waterspouts of this mild variety infest 
all tropical seas to which thunderstorms are common, 
but they seldom last more than 15 minutes. You’ll 
see many of these “spouts” while flying over tropical 
seas. Remember that, though mild in relation to their 
terrific cousins, they are unfriendly to aircraft. 

The South Sea Islands 

The islands lying east of Australia between the 
equator and 25 0 S. Latitude are the Southern Hemi¬ 
sphere counterpart of the island groups described 
in the previous section, and climatically are quite 
similar to them. The Southern Hemisphere islands, 
however, are more numerous, larger and more 
mountainous than those north of the equator. 



MANY STORMS OF THE TROPICS ARE INSULAR 

RATHER THAN MARINE PHENOMENA 


Therefore, since many of the thunderstorms and 
squalls of the tropics are insular rather than marine 
phenomena, the South Sea Islands experience more 
intense and more frequent storms than the smaller 
islands north of the equator. During the day the large 
islands get hot enough to cause many daytime 
thunderstorms (Figure 147). During the night they 
cool off enough in places to cause strong land 
breezes and katabatic mountain winds that produce 
squalls off the coasts. 

West of i6o°W. Longitude the intertropical front 
swings southward over the South Sea Islands for the 
southern summer, getting farthest south in March 
(Figure 128). In January the intertropical frontal 
zone lies over the Phoenix, Ellice and Solomon Is¬ 
lands. In March, (farthest south) it’s over the 
Samoa, Fiji and New Hebrides groups. By April it’s 
back on the Phoenix-Ellice-Solomon line—then it re¬ 
treats northward for the northern summer. 

Each island group has its worst flying weather 
when it is visited by the intertropical front. The 
Tonga Islands, for example (2o°S. Latitude, i75°W. 
Longitude), have trade wind weather most of the 
year. From December to March, however, they re¬ 
ceive occasional visits of the intertropical front and 
periods with doldrum conditions. This causes the 
average March to have 20 rainy days, while June, by 
contrast, has only 12 rainy days. 

Throughout the South Seas region, thunderstorms 
are most common in summer, and increase as you 
approach the equator. Tonga Islanders can expect 
about 20 thunderstorms a year, most of them in the 
summer season (December to March). Fiji Islanders 
can expect thunderstorms on about 38 days a year, 
also with a summer maximum. Nearer the equator, 
Rabaul, on New Britain, has an average of 73 thunder¬ 
storm days a year, with November the most thun¬ 
dery month. Many of these tropical thunderstorms 
develop along the intertropical front. Some are oro¬ 
graphic, raging on windward mountain slopes. Others 
are air mass storms that form in doldrum or near- 
doldrum conditions, and are caused by high tempera¬ 
tures and by weakening or absence of the trade 
winds. 

The tropical cyclones of the South Pacific occur 
chiefly among the Fiji, Tonga, New Hebrides and 
Samoa Islands. Of 119 recorded cyclones, 104 oc¬ 
curred from December to March inclusive, and none 
between June and August. 

Flying conditions are at their best in the winter 
or dry season, except where adverse local conditions 
obtain. Visibility often is lowered in the area between 
the coasts of Australia and New Guinea by haze and 
by smoke from brush fires. Active volcanoes are the 



Fig. 147. Cumulus clouds forming over the islands 
of the Solomons (U.S. Navy photo) 

cause of thick haze in the vicinity of Rabaul, where 
there are six active volcanoes within seventy-five 
miles of each other. 

Contact flight may be maintained generally during 
the dry season except along windward coasts. In 
these coastal areas exposed to the full sweep of the 
southeast trades, low stratus clouds form along the 
coast and offshore. Ceilings under the stratus are 
usually 500 to 700 feet, but occasionally they lower 
to 200 to 300 feet. Severe turbulence and ceilings 
near zero occur over high mountain sections. The 
tall Owen Stanley Range of New Guinea, over 
13,000 feet high, occasions a severe local wind known 
as “the Gula”. This is a katabatic wind and occurs in 
the early morning. It hits Port Moresby five or six 
times a year and often reaches velocities of 60 to 70 
miles per hour, and lasts 20 to 30 minutes. It can 
cause great damage to aircraft on the ground. 

New Zealand and Eastern Australia 

In southeastern Australia you will encounter flying 
weather that will remind you of the central Atlantic 
coast of the United States. In New Zealand you’ll 
be reminded of the Pacific Northwest. Westerly 
winds, mP and mT air masses, polar fronts and extra- 
tropical cyclones cause these similarities. Tropical 
cyclones, too, may occasionally strike the east coast 
of Australia and northern New Zealand, just as they 
may affect the eastern coast of the United States. 


147 


Like our own west coast, New Zealand faces the 
west winds with high mountain ranges, and rainfall 
is closely correlated with topography. Heavy pre¬ 
cipitation, exceeding ioo inches a year, falls on some 
windward slopes, while in some rain-shadows on the 
east slope of South Island precipitation is as low as 
20 inches. 

Eastern Australia has more uniform rainfall than 
does New Zealand. In summer the SE. trade winds 
bring rain to the east Australian coast (compare with 
east coast of United States), and in winter the stormy 
westerlies bring rain. The main difference between 
southeastern Australia and east central United States 
lies in the fact that there is no large land area pole- 
ward of Australia, hence it does not experience 
northers or winter cold spells. The nearest approach 
is the “southerly buster”, but this is a blast of mP 
air and not cold cP air. Thus, summers are less hot 


and winters are less cold in southeast Australia than 
in east and central United States. 

General flying conditions along the coastal routes 
are good. Clouds are usually broken or scattered, and 
visibility is good, with persistent fog rare. Thunder¬ 
storms are more numerous in North Island and the 
Australian coast near Sidney than farther north. 
These are cold front storms and occur most fre¬ 
quently in winter. Snow and severe hail rarely 
fall except in the mountains and on the southern 
part of South Island. 

Probably the greatest handicap is strong winds— 
gales (above Beaufort 7). Wellington experiences 
gales on about 60 days a year. Gustiness is also preva¬ 
lent, especially in the straits between the New Zea¬ 
land islands. 

Both Australia and New Zealand have well 
equipped airports with meteorological service and 
radio facilities. 


1 

SOUTHWEST PACIFIC AIMD INDIAN OCEAN 


The island-studded ocean area lying between Australia and Asia, extending from New Guinea and 
Guam westward to India and Ceylon, may be considered a unit both strategically and climatically. Stra¬ 
tegically, it includes all southern naval approaches to Japan. Climatically, it is controlled by monsoons 
(Figure 149). 


THE MONSOONS AND THE MIGRATION OF THE 
INTERTROPICAL FRONT 

“Monsoon” means seasonal. The monsoon is a 
wind that persists from the same direction for long 
periods and then undergoes a complete reversal with 
change of season. Summer and winter monsoons 
bear the same relations to summer and winter that 
the localized sea and land breezes bear to day and 
night. Summer monsoons, like sea breezes, blow 
toward the land when the land is heated (blow 
toward low pressure over the heated land). Winter 
monsoons, like land breezes, blow from the land 
when the land is cooled (blow fro? 7 i high pressure 
over the cooled land). Land and sea breezes are local 
developments along tropical coasts; they seldom ex¬ 
tend far from shore and seldom influence the air 
currents deeper than 1000 to 3000 feet. Monsoons 
are built on a bigger scale. They respond to the sea¬ 
sonal heating and cooling of the great land mass of 
Asia (Figure 150). 

Monsoons of the Northern Winter 

Consider first the months of January and Febru¬ 
ary-winter in the Northern Hemisphere. At this 
time of year the Asiatic continent is occupied by an 
intense anticyclone or high of very cold air. The 
mean temperature in this anticyclone ranges from 
about — 2o°F. to -6o°F. During these months the 
“down under” continent of Australia has summer 
weather with average temperatures about 85 °F. to 
90 °F. Low pressure develops in the summer-hot air 
over the northern part of Australia. You will recog¬ 
nize this low as part of the doldrum belt, which ex¬ 
tends farthest south at this season. Figure 150 shows 
how, with high pressure over cold Asia and low 
pressure over hot Australia, the air that influences 
the southwest Pacific flows outward from the Asiatic 


high and inward toward the Australian low. While 
in the Northern Hemisphere, it swings into and 
becomes a part of the NE. trade winds. Sweeping 
from the northeast across the Philippines and onward 
toward the equator, it carries the monicker north¬ 
east monsoon. Crossing the equator, it is deflected 
toward its left (like any well-behaved Southern 
Hemisphere wind) and moves toward Australia as 
the northwest monsoon. 

During the winter of the Northern Hemisphere, 
while monsoon winds blow from Asia to Australia 
across the Southwest Pacific, monsoon winds also 
blow outward from India across the North Indian 
Ocean. The low-pressure goal of the Indian winter 
monsoon is the doldrum belt, or intertropical front, 
south of the equator over the Indian Ocean. Across 
the Bay of Bengal and the Arabian Sea, as in similar 
latitudes in the Southwest Pacific, the winter mon¬ 
soon blows as a northeast wind. 

Monsoons of the Northern Summer 

Consider now the months of July anad August- 
summer in the Northern Flemisphere. Conditions 
are exactly reversed. The intertropical front has mi¬ 
grated far north and a great low-pressure area or 
belt occupies much of warm southern Asia. Aus¬ 
tralia, in contrast, experiences winter. With the win¬ 
ter land cooler than surrounding seas, high pressure 
prevails over the Australian interior. During this sea¬ 
son, then, air flows across the Southwest Pacific from 
Australia to Asia. While still south of the equator, 
this air flows as the southeast monsoon. North of the 
equator, as between Singapore and the Philippines, 
it deflects to its right and becomes the southwest 
monsoon. North of the Philippines and along the 
China coast, this air deflects again; drawn by the 
Asiatic low, it pours onto the continent from the 
southeast. 


149 



20 ° 



KEY 

Over 10,000 ft. 
5,000 - 10000 ft. 
1,000 - 5,000 ft. 
Under 1,000 ft 


O 


C 


S 


70' 


80‘ 


90‘ 


100 ' 

















1 f0‘ 


120 ° 



130 e 


140‘ 


. 

HonQ TaKao* ^ 


/ 


£ 

/ 


, 4 - 


BONIN IS. 


zcr 



sour 

H 0 tf 

v 

CHIN 

N * 
D 

A > % 


SEA 

* 

J 



> - 


or 

4 ' 

<?o n 




PA C / F / C 


OCEAN 


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GUAM 


10 ° 




'V * 1 

+ 0 * * Davao 


'O- 




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Fig. 149. Locational and physical map of the Southwest Pacific and Indian Ocean 


151 






















Over the Indian Ocean, summer monsoon winds 
flow from the subtropical high of the Southern 
Hemisphere toward the Asiatic low (Figure 150). 
These monsoon winds deflect as in the Southwest 
Pacific; that is, south of the equator they blow from 
the southeast, and north of the equator they blow 
from the southwest. What, then, is the prevailing 
direction of summer winds in the Bay of Bengal? 

The Transition Seasons 

Figure 151 shows the average location of the inter- 
tropical front through the Southwest Pacific and 
North Indian Ocean and also shows the general sur¬ 
face wind pattern of the area for the months of Feb¬ 
ruary, May, August and November—months repre¬ 
sentative of the four seasons. The transition from 
one monsoon season to the other is gradual and 
fluctuating. 

With the coming of the Northern Hemisphere 
spring, the Asiatic high weakens, so the winter mon¬ 
soon weakens. By May most of the doldrum and 
intertropical front activity of the Southwest Pacific 
has moved northward from Australia to near the 
equator. Air currents flow to this belt from both 
hemispheres, but throughout a wide equatorial belt 
winds are both weak and variable. In the spring tran¬ 




sition season, winds over the Bay of Bengal are also 
weak and variable in comparison with the strong 
steady winter monsoons. By July and August the 
intertropical front has moved far north. Summer 
monsoons then blow quite steadily over both the In¬ 
dian Ocean and the Southwest Pacific. 

With the coming of the Northern Hemisphere 
autumn and the filling of the Asiatic low, the sum¬ 
mer monsoons weaken. In September northeast 
winds set in over the northern portion of the Philip¬ 
pines, with southwest winds over the rest of the area 
north of the equator. The intertropical front is mov¬ 
ing southward. In October and November the north¬ 
east winds progress gradually southward but become 
light and variable below 5°N. Latitude. By Novem¬ 
ber conditions approximate those of May, with dol¬ 
drum and intertropical front activity near the equa¬ 
tor. Air currents again flow to this belt from both 
hemispheres, with most winds weak and variable 
throughout a wide equatorial belt. By December the 
northeast monsoon wind is strong and steady over 
the Philippines; near the equator it weakens and be¬ 
comes a northerly monsoon; passing south of the 
equator, it becomes a northwest monsoon. The air 
currents of the northern winter then attain full force 
and dominance over the Southwest Pacific. 



Fig. 150. Monsoon winds between India and Australia 

152 






























AVERAGE 

POSITION 

OF 

INTERTROPICAL 

FRONT 


FEBRUARY 


MEAN POSITION 
OF FRONT 


DAY BY DAY 
MOVEMENT OF FRONT 


MAY 


SUBSIDIARY 
SQUALL ZONE 


AUGUST 


NOVEMBER 



Fig. 15i 


153 


































RAINY SEASON 


DRY SEASON 


AUSTRALIAN LAND MASS 


ASIATIC LAND MASS 


Over the Indian Ocean a similar transition has 
taken place. Outline in your mind this seasonal 
change over the Indian Ocean. 

Pilots consider the entire monsoon region to have 
“good” flying weather. But “good” is a compara¬ 
tive word. The flying weather of Texas is “good” 
when compared with that of New England, but a 
pilot can hit difficult weather over Texas. So it is 
in the Southwest Pacific and Indian Ocean. You’ll 
find many more good flying days than off New¬ 
foundland or the Aleutians, but to keep out of 
weather trouble, and (even more important for a 
Navy flyer) to use all of the aids weather offers to 
patrol and combat, you should understand the “road 
conditions” of your monsoonal airways. Properly 
understood, the weather in this area generally will 
be your good friend and will serve you well. 


THE SEASONAL PATTERN OF WEATHER 

The movement of the monsoons and the accom¬ 
panying movement of the intertropical front deter¬ 
mines the prevailing surface wind directions. But 
even more important to pilots, the monsoons are 
moving air masses. The nature of these air masses, 
their modifications over tropical seas and lands, and 
their behavior along fronts, produce the flying 
weather you will encounter when flying in this vital 
area. 


Seasonal Pattern of Air Mass Weather 

In Winter of the Northern Hemisphere, the 
air that pours out of Asia across China and India, is 
polar continental (cP). Over Asiatic coasts it is 

154 


cool, dry, and stable, bringing clear skies.* Moving 
over the warm ocean surface, it warms in lower lay¬ 
ers and soon develops scattered cumulus or patches of 
strato-cumulus clouds. Moving equatorward as a 
northeast monsoon, warming and picking up much 
moisture, the cP air modifies to tropical maritime 
(mT) air with increasing amounts of cumulus 
clouds. Then, over the warm equatorial seas, it modi¬ 
fies to equatorial (E) air. This equatorial air is moist 
to great heights. Unstable, it develops towering 
cumulus and thunderstorms on the slightest provoca¬ 
tion. Windward coasts become cloudy, as Asiatic 
air that has picked up moisture over the seas pours 
toward the Southern Hemisphere. The amount of 
windward cloudiness increases as the air becomes 
progressively more moist. Moving south of the 
equator as a northwest monsoon, the equatorial air 
advances to the belt of doldrum or intertropical 
front activity, which in this season extends across 
northern Australia and the South Indian Ocean. 

The monsoon that blows from Asia, as outlined 
above, produces a dry season over the northern por¬ 
tion of the monsoon area and a rainy season over the 
southern portion. During the period of December 
through April, little rain falls along the coasts of 
Burma and India and over the northern part of the 
Bay of Bengal. Polar front activity causes some win¬ 
ter precipitation over the seas north of the Philip¬ 
pines, but the northeast monsoon brings relatively 
dry weather (with scattered clouds) to the seas be¬ 
tween the Philippines and the East Indies. Farther 
south, convectional rainfall increases, reaching a 
maximum in doldrum conditions south of the equa¬ 
tor. 

Throughout the entire sweep of the winter mon¬ 
soons, mountainous windward slopes have orographic 
cloudiness and precipitation. In general, this wind¬ 
ward wetness increases to the southward, but even 
the windward (northeast) coasts of the Philippine 
Islands are rain-drenched in the season of the north¬ 
east monsoon. In contrast, Manila on a southwest 
Philippine coast, has its dry season. 

In Summer of the Northern Hemisphere, the 
air that pours out of Australia is tropical continental 
(cT)—dry and stable. Moving toward the equator 
as a southeast monsoon, this air brings a dry season 
to the waters north of Australia and to the southern¬ 
most East Indies. Little or no clouds form over the 
sea, and only fair weather daytime cumulus forms 
over the leeward slopes of islands. While moving 


* Except where disturbed by polar front activity, common in 
winter off the China coast. 

















over the ocean, however, the dry Australian air 
j picks up some moisture, and windward slopes be¬ 
come clouded—even the windward slopes of Timor, 
not far from the Australian air mass source. As this 
air gets farther from Australia, crosses the equator 
and moves into the Northern Hemisphere as a south¬ 
west monsoon, it becomes progressively more moist. 
Not only does the migrating air become more moist, 
but also thermal convection increases to the north¬ 
ward, for the heat equator has migrated into the 
Northern Hemisphere. Thermal convection makes 
the moist air boil up to produce scattered cumulus 
clouds and occasional showers. Cloudiness and pre¬ 
cipitation on windward slopes also increase as the 
air moves northward. The northern portion of the 
monsoon area now has its rainy season. 

The air that in this season moves north to the 
coasts of India and Burma originates as mT air in 
the subtropical high of the South Indian Ocean. Like 
the air that moves across the Southwest Pacific 
from Australia, it acquires equatorial properties on 
crossing the warm equatorial waters and blows 
across the Bay of Bengal as a warm moist southwest 
summer monsoon. It becomes increasingly clouded 
as it moves north, and brings heavy “monsoon sea¬ 
son” rains to India and Burma. 

During the northern summer then, there is a dry 
season near northern Australia and across the Indian 
Ocean south of the equator. Tropical instability rains 
(showers) increase to the northward. Windward 
slopes are clouded and rainy throughout the entire 
sweep of the monsoons. India, Burma, China, and 
the Philippines have their rainy season. 

Transition Seasons. In the weak and variable 
winds of the transition seasons (April-May and Oc- 
tober-November) between the seasons of winter 
and summer monsoons, convective activity becomes 
strong in the slowly-stirring air along a wide equa¬ 
torial belt. The air that moves into this belt from 
both hemispheres becomes increasingly warm, moist, 
and unstable, as it nears the equatorial region. In 
these transition seasons, then, the most difficult flying 
weather lies in the belt along the equator. 

Seasonal Pattern of Frontal Weather 

You have seen how the belt of intertropical front 
activity makes its seasonal migrations across this 
region. From its southernmost position in January- 
February to its northernmost position in July- 
August, it migrates farther in this monsoon area than 
anywhere else in the world. This, as you know, is 
because of the summer heating of the great Asiatic 
land mass, which draws the intertropical front far 



Fig. 152. Map of cold-front—warm-front type doldrum 
as reported by KNILM pilot 


north, and the winter cooling of that land mass, 
which produces an outflowing of air that drives the 
intertropical front far south. 

Weather in the Intertropical Front. Except 
for its greater latitudinal migration, the intertropical 
front of this region is no different from the inter¬ 
tropical front of other tropical regions. It brings the 
same types of weather as in the tropical Atlantic and 
Central Pacific. At any one place it may exist as a 
doldrum belt or as a sharply defined front, and 
frontal weather may develop where doldrum condi¬ 
tions existed a few hours before, or vice versa. 

During the transition season between monsoons 
(that is in April-May and again in October-Novem- 
ber) the wide equatorial belt has frequent “spells” of 
doldrum weather—light and variable winds with 



Fig. 153. Cross section of cold-front—warm-front type 
doldrum as reported by KNILM pilot 


155 















moist air and many convectional showers and air 
mass thunderstorms. This zone moves north for 
the northern summer, south for the southern sum¬ 
mer. 

The doldrum belt weather may have any of the 
variations outlined for doldrum weather in the Central 
Pacific (pp. 136-138). An example of type C (Figure 
138) is revealed in the reports of a Royal Dutch Air¬ 
lines pilot flying from Singapore to Saigon. He 
found the two edges of a doldrum area closing in. 
These two edges, roughly parallel and only 50 miles 
apart, lay NW.-SE., with southwesterly winds to 
the windward of the southwestern edge and easterly 
winds to the windward of the northeastern edge 
(Figure 152). The pilot crossed this doldrum area 
on a track of about 040 0 true. The aircraft ran first 
through a narrow belt of moderate rain followed by 
heavy rain and thunderstorms; then it came out into 
very light drizzle from the second “edge” (Figure 
153). There was a very definite falling off of wind 
in the section between the two edges. 

The coming of winter in one of the hemispheres 
creates vigorous frontal activity along the intertropi- 
cal front. As winter develops in one of the hemi¬ 
spheres, the flow of air from that hemisphere drives 
the front into the other hemisphere. Every advance 
of the front produces squally weather. With the 
northward advance of the southwest monsoon in 
May and June, for example, the Bay of Bengal has 
disturbed, squally weather and much rain. During the 
squalls, ceilings may descend to 300 feet and thunder¬ 
storms may rage. Airmen who know the Bay of 
Bengal have divided the squalls (by appearance) into 
white, brown, and black squalls. It is said that white 
squalls may be flown through, while brown and 
black squalls are dangerous. Some advance as long 
line squalls, but most of them can easily be flown 
around. 

Doldrum and intertropical front activity, then, 
with its seasonal migrations in response to the migra¬ 
tions of the heat equator, produces squally weather 
and many thunderstorms north of the equator during 
the northern summer and south of the equator dur¬ 
ing the southern summer. It produces a wide belt of 
thunderstorm weather throughout equatorial regions 
during the transition season. Thus, it reinforces and 
accentuates the seasonal weather pattern caused by 
the air mass characteristics and modifications. 

Invasions of Polar Front Activity. The SW.- 
NE. extending polar front that forms in winter 
between the cold cP air from Asia and the warmer 
mT air from the Pacific subtropical high frequently 
lies between Japan and the Philippines. Strong out¬ 

156 


bursts of cP air sometimes drive this front south¬ 
eastward across the Philippines or even beyond them 
to the southeast. Here, near its equatorward end, 
this front is only feebly developed. It resembles the 
trough at the equatorward end of the Atlantic polar 
front—a trough that sometimes crosses the Caribbean 
as the front of a “norther” or invasion of polar air 
from North America. Clouds form and ceilings 
lower where the colder air undercuts the warmer, 
but little turbulence develops. The cyclonic waves 
that move northeastward along this polar front gen¬ 
erally form or gain their force farther north. This 
trough, however, measurably influences the winter 
flying weather of the northern Philippine area and 
the Guam-Manila-Hongkong air route. 

An occasional “western disturbance” invades 
northern India in the winter season. These “western 
disturbances” are extratropical cyclonic storms that 
originate along the Mediterranean front and move 
eastward to advance finally across the Ganges plains 
south of the Himalayas. Most of the winter storms 
of the Mediterranean dissipate long before reaching 
so far east. Only about two “western disturbances” 
cross India in January, the month of maximum 
occurrence. They cause light rains over the plains 
and some thunderstorms over the hills and over the 
northern part of the Bay of Bengal. They make a 
slight but sometimes significant change in the other¬ 
wise clear flying weather of the season of winter 
monsoons. 

Tropical Cyclones 

Tropical cyclones are everywhere alike or similar. 
Terrors of the tropics, they differ only in name. 
Those of the Atlantic and Caribbean are known as 




hurricanes, but in the area of the Southwest Pacific 
and the Indian Ocean, they have three different 
designations. In the Bay of Bengal and the Arabian 
Sea they simply are called “cyclones”; over the 
China Sea and near Japan they are known as “ty¬ 
phoons”; over the Philippines they may be called 
either “typhoons” or “baguios”; and off the north¬ 
west coast of Australia fliers know them as “willy- 
willies”. 

As in the Atlantic, so in the Southwest Pacific and 
Indian Oceans, the tropical cyclones are of seasonal 
nature, having their greatest frequency toward the 
end of the hot season. The following table shows, 
for each area, the average number of tropical 
cyclones reported per year. Note that they are most 
frequent in the China Sea and Philippine area. 

To avoid the tropical cyclones of the China Sea 
and Philippine area, the Bay of Bengal and the Ara¬ 


bian Sea, you apply the rules that you learned in 
Chapter y. To avoid the “willy-willies” of north¬ 
west Australia and other “cyclones” of the South In¬ 
dian Ocean, remember that they are Southern Hemi¬ 
sphere storms and behave like the tropical cyclones 
of the South Sea Islands described in Chapter 7. 

The important things for you to know are: 

(1) The signs of the approach of a tropical 
cyclone, which are the same the world over. 

(2) The periods of most frequent occurrence, 
shown in the table below. 

(3) The characteristic paths of the storms, shown 
on the map, Figure 154. 

Above all, do not consider the fact that typhoons 
are not described in detail in this chapter an indica¬ 
tion that these storms are not important in the mon¬ 
soon area. They are more frequent and more de¬ 
structive here than anywhere on earth! 


FREQUENCY OF TROPICAL CYCLONES OF THE SOUTHWEST PACIFIC AND INDIAN OCEAN 


Region 

Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

Year 

Arabian Sea. 

.02 



.04 

.12 

.13 

.04 


.01 

.12 

.15 

.04 

•67 

Bay of Bengal. 




.12 

.24 

.06 

.09 

•03 

.15 

.27 

.90 

.12 

1.98 

China Sea and Philippines. 

1.20 

.68 

.72 

•54 

1.27 

1.30 

3.48 

3.55 

4.25 

3.67 

2.00 

1.30 

23.96 

So. Indian Ocean*. 

1.31 

1.70 

1.22 

.53 

.19 





.06 

•25 

.78 

6.04 


•Including: (i) the “willy-willy” region off the northwest coast of Australia, and (2) the tropical cyclone region in the 
western portion of the South Indian Ocean off the coast of Madagascar. 


157 
























































EFFECTS OF LAND AND SEA CONTRASTS 
AND MOUNTAIN BARRIERS 

The general weather conditions you will en¬ 
counter on a flying mission in any portion of the 
moonsoon region will depend upon the season— 
upon the general pattern of air mass movements 
and modifications and the frontal locations during 
that season. Anywhere within the monsoon region, 
the general seasonal pattern of weather may be 
altered by local effects—by effects of mountain 
barriers and of temperature contrasts between land 
and sea. 

Land and Sea Breezes 

Almost everywhere in the tropics, land and sea 
breezes influence flying weather. In this monsoon 
region they are a regular feature of the daily 
weather on the coasts of all large islands and on con¬ 
tinental coasts. During the transition periods when 
monsoon winds are weak, land and sea breezes domi¬ 
nate the circulation along these tropical coasts, with 
the sea breezes (owing to the dominance of the sea) 
generally stronger and deeper than the land breezes. 
Even when monsoons are strongly developed, land 
and sea breeze tendencies strengthen or weaken them. 
The sea breeze tendency, operating by day, 
strengthens the monsoon on windward coasts and 
weakens it on leeward coasts. The land breeze ten¬ 
dency, operating by night, weakens the monsoon on 
windward coasts and strengthens it on leeward coasts. 
On windward coasts, then, moonson winds are 
strongest during the day; on leeward coasts monsoon 
winds are strongest during the night. A striking ex¬ 
ample occurs on the north coast of Sumatra, at the 
beginning and ending of the southerly monsoons in 
April and October. Heavy squalls called “Sumatras” 
sometimes occur at night, with winds rushing off the 
coast and attaining velocities up to 50 knots. These 
“Sumatras”, and similar squalls elsewhere induced by 
land and sea contrasts, are the exception.* Generally, 
land and sea breezes average only 8 to 15 knots. They 
are usually only 1000 to 3000 feet deep and extend 
influences only a few miles from shore. 

Katabatic Winds 

The “Sumatras” and similar squalls caused by 
cooled night air rushing from cool land to warmer 
sea, are land breezes—but they are more than that! 
Most of these nighttime land breezes that attain 

* The most severe “Sumatras” occur when an intertropical 
front is moving from land to sea across a mountain range. The 
squall is then produced by the cool frontal air surging down the 
mountain slope onto the sea. These, therefore, are not entirely 
induced by land and sea temperature contrasts. 


great force are also katabatic winds (fall winds). In 
this region many mountains and plateaus stand close 
to the sea. When air over a plateau becomes cooled 
by nighttime cooling, it can flow down the slope as 
a wind that may attain great force. On leeward coasts 
where such winds (coming off mountains) combine 
with the land breeze influence, the combination mon¬ 




soon—land breeze—fall wind can attain enough force 
to be a great hindrance to flying. Also, in some places 
far from sea (as on leeward slopes in high passes of 
the Himalayas) the fall wind effect lends great force 
to winter monsoons. 

Orographic Fronts 

When an air mass approaches the equator, say, as 
a northeast wind, there is a tendency for it to deflect 
to a northerly wind which finally crosses the equator 
and becomes a northwest wind. When such an air 
mass, while still blowing from the northeast, crosses 
a land mass, there is slowing down of the air stream 


158 




















in the lower layers because of increased friction over 
the land and the tendency to turn or deflect is in¬ 
creased. If this diverted air mass meets an air mass 
that is still undiverted because its track has been over 
the sea, a “front” will sometimes form at their meet¬ 
ing place (Figure 155). 

Figure 156 shows a slightly different type of 
orographic front—different in cause only. The large 
island of New Guinea contains extensive mountain 
ranges extending to an average of some 10,000 feet 
in height. These mountains act as a natural barrier to 
the flow of air. Figure 156 shows how, in the season 
of the northwest monsoon, a portion of the monsoon 
sweeps around to the south of New Guinea as a 
westerly wind. At the same time a portion of the 
monsoon (or possibly trade air) moves past the east 
end of New Guinea as a northerly wind. Where the 
two air streams meet you’ll find a “front”. 

Pilots flying over the island-strewn Southwest 
Pacific may encounter several orographic fronts 
within a distance of 150 or 200 miles. The depth of 
each of these fronts is about the same as the height of 
the land area involved (usually not over 4000 or 
5000 feet). Often the front does not contain much 
turbulence or rain; if the overrunning air mass is 
slightly unstable, however, the extra lift produces 
thunderstorms. The ceiling may be pretty close to the 
water and you may be forced to fly very low. Some 
of these fronts have considerable wind shifts, pre¬ 
senting problems relative to navigating. To navigate 
efficiently in these situations it becomes necessary to 
watch closely the ripples on the surface of the sea. 

Each orographic front is limited in extent and lies 
entirely within the single air mass. It remains more 
or less stationary while the wind is steady, but 
changes its position or even dissipates with a change 
of wind direction. 

The type of orographic front shown in Figure 157 
occurs in the hemisphere having its summer season. 
The type shown in Figure 156, however, occurs in 
the hemisphere which is having its winter season. 
In the region from the Philippines to the equator, 
fronts of this type form frequently during the sea¬ 
son of the northeast monsoons—also in the Malacca 
Straits and near islands west of Malaya. 


Windward Coast Cloudiness 

Much cloudiness and frequent low ceilings char¬ 
acterize mountainous windward coasts throughout 
the region invaded by monsoon winds of moist equa¬ 
torial air. Thus, north of the equator, clouds drape 
northeast coasts during the season of northeast mon¬ 
soons, and drape southwest coasts during the season 
of southwest monsoons. South of the equator, the 


northwest monsoons condense much moisture on 
northwest slopes, while in the opposite season the 
southeast monsoons carry some (but less) moisture 
to obscure the coastal mountain ranges facing south¬ 
eastward. 

Leeward coasts are more frequently clear. Cumu¬ 
lus clouds may form on leeward plains during day¬ 
time hours, especially when air is moist and when 
a strong sea breeze tendency weakens (or neutral¬ 
izes) the monsoons, producing stagnant air condi¬ 
tions conducive to convection. 

The clouds of windward coasts will be stratus if 
the air is stable but will include towering cumulus 
and thunderstorms if the air is unstable. The clouded 
area commonly extends from the mountains to the 
coast and from the coast seaward for a distance of 
about 50 miles. Ceilings near the outer margins of the 
clouded area are commonly 1500 to 3000 feet; near 
the coast they commonly lower to 700, 500, or even 
200 feet. Rain squalls accompany these low clouds 
on windward coasts and visibility may be reduced to 


NOTE ■- Arrows indicate dominant direction of force of winds at the levels indicated. 
Read the wind arrows as you read direction on a map North to top, South to 
bottom. West to left, East to right. Thus an arrow pointing up) means a 


South wind, notan air stream moving vertically. 






Sea Level — 



Northeast j Winds 

(Trades an<Tfiorthers) 

_ 

10*N. 


WINDS ALOFT - JANUARY 
Fig. 157 



Fig. 158 


159 






























one-fifth mile. The tops of the clouds along wind¬ 
ward coasts usually will be found between 6000 and 
8000 feet, but the clouds may extend upward over 
mountains. Such clouds are always “stuffed with 
rocks”. As a rule, pilots prefer contact approaches 
to bases under the clouds. 

Diurnal Changes in Storminess 

The squalls that frequent these tropical seas are 
most active and severe at night. During daylight hours 
a squall commonly remains comparatively mild or 
may disappear altogether. During the night it may 
develop turbulent activity and boil up into thunder¬ 
storms. 

This is the most thundery region on earth. Some 
island stations average more than 300 thunderstorms 
yearly. Batavia, Java, receives its annual 72 inches of 
rainfall (over twice the precipitation of Chicago, Illi¬ 
nois, or Seattle, Washington) in 350 hours (averaging 
about one hour of rain a day), mostly in thunder¬ 
storm downpours, with clear weather prevailing at 
other times. Over the land, as at Batavia, most thun¬ 
derstorms occur during the day. Over the sea most 
thunderstorms occur during the night. Over coasts 
they are more impartial to the clock. 

Frontal thunderstorms form along the intertrop- 
ical front, migrating with the seasons and becoming 


/F YOU MUST DO TH/S- 



most intense over the seas at night. Orographic thun¬ 
derstorms form on windward slopes. When a strong 
monsoon blows, thunderstorrrt activity may occur on 
windward slopes during day or night; but more com¬ 
monly night-cooling of the land and night breezes 
from land to sea drive thunderstorms to the coasts 
during evening hours and onto the sea during night¬ 
time. Air mass thunderstorms abound throughout 
this area, especially in the summer hemisphere dur¬ 
ing monsoon seasons and in a wide equatorial belt 
during the transition seasons. As with thunderstorms 
of other types, cumulo-nimbus clouds and squally 
turbulence frequent the seas by night and the islands 
by day. 



160 


Fig- 159- Map of winds at 10,000 feet over SE..Asia, January 















GENERAL SUMMARY OF FLYING CONDITIONS 

In general you will find daytime flying conditions 
of any section of the monsoon region to be fair to 
good during the rainy season of that section and ex¬ 
cellent during the dry season. Excessive nighttime 
turbulence in fronts and in thunderstorms makes 
night flying difficult during the rainy season. 

Except in the vicinity of a severe tropical cyclone 
—which you should avoid—contact flights can be 
made over the seas. Air mass showers and thunder¬ 
storms seldom cover an area of more than 20 to 24 
miles and can be evaded. Frontal squalls, when they 
cannot be easily flown around, can be flown under. 

Ceilings and Visibility 

Except near windward coasts and in severe squalls, 
ceilings rarely drop to below 500 feet. Layers of 
middle or high clouds may be present near the inter- 
tropical front, and warming cP air may have patches 
of strato-cumulus, but (except in limited areas of 
windward coasts) continuous layers of low cloud 
never cloak these tropical seas. 

Dust haze sometimes lowers visibility to 3 to 6 


miles over seas north of Australia when the dry mon¬ 
soon blows from Australia, while smoke from brush 
fires may lower it locally to one mile. Similarly, dur¬ 
ing the season when the monsoon blows from Asia, 
dust from Asia sometimes affects visibility over the 
China Sea and the Bay of Bengal. When flying at 
6000 feet, dust haze can seriously blur your view of 
the surface. 

Over these warm-water seas, fogs never form. 
Along mountain sides and over plateaus at levels of 
3000 to 7000 feet, upslope or radiation fogs may 
develop during the night and early morning, dissi¬ 
pating before noon. Also, clouds frequently hide 
mountains. Except for the ‘'haze nuisance, however, 
and the fog or clouds against mountains, visibility is 
good throughout the entire monsoon region. 

Winds 

Every pilot in this area must know the effects of 
islands and coasts on wind direction. Except for the 
land and sea breezes, the monsoonal wind pattern re¬ 
mains remarkably dependable during the height of 
the monsoon seasons. Transition seasons, as you 
know, produce variable winds throughout a wide 



Fig. 160. Map of winds at r0,000 feet over SE. Asia, July 


161 



















equatorial belt (io°N. to io°S.), with easterly trade 
winds flowing into this belt from both hemispheres. 

Winds Aloft. You may seldom fly above the sur¬ 
face layers of the monsoon. It is well to know, how¬ 
ever, the way in which minds aloft fit the monsoon 
pattern. 

During January-February the monsoon current 
moving out of Asia is about 5000 feet deep at Lati¬ 
tude 20 °N. From the surface to the top of this air 
flow, the winds become more and more easterly. This 
flow of monsoon winds, becoming easterly aloft, in¬ 
creases in depth from about 5000 feet at 20°N. Lati¬ 
tude to about 25,000 feet at io°N. Latitude (Figure 
157). Above the easterly air current westerly winds 
blow. A transition zone of variable winds aloft be¬ 
tween easterly and westerly winds, therefore, slopes 
from about 5000 feet at 20°N. Latitude to 25,000 
feet at io°N. Latitude. Beneath that zone the easter¬ 
lies blow, above it the westerlies (Figure 157). Fig¬ 
ure 159 shows the situation at 10,000 feet. 

Near the equator, where surface winds of Janu¬ 
ary-February change from northeast (in Northern 
Hemisphere) to northwest (in Southern Hemi¬ 
sphere), winds aloft are variable (Figure 157). 
South of the equator in this season the north¬ 
west monsoon air current becomes westerly aloft 
and extends to above 10,000 feet (Figure 157). No¬ 
where in the monsoon region are wind directions 
aloft as dependable as they are in the middle latitudes. 

You have learned (and Figure 157 shows) that the 
monsoonal flow of cold air that leaves Asia in Janu¬ 
ary-February is swift but shallow (only about 5000 
feet deep) at 20°N. Latitude. You have learned that 
it slows down, modifies to equatorial air, and grows 
much deeper as it approaches the equator, and that 
from south of the equator to io°S. the dominant air 
movement up to 10,000 feet is from the west. Note 
also on Figure 157 that wind velocity aloft increases 
in the monsoon current only to decrease with ap¬ 
proach to the zone of variable winds aloft. In the 
upper westerlies wind velocity increases with in¬ 
crease in height. 

During the opposite season, July-August, the mon¬ 
soonal flow from the Southern Hemisphere, warmer 
and somewhat deeper even at its source than the 
January-February monsoon, approaches Asia as a 
deep current—about 15,000 feet deep. Up to about 
15,000 feet, wind directions are essentially the same 
as at the surface (Figure 158). The easterly winds 
south of the equator become variable at the equator, 
and deflect to become southwesterly winds from the 
equatorial area to beyond the Philippines. Along the 
monsoonal coasts of China the southeast surface wind 


of summer is overlaid by a southwest wind at about 
5000 feet. Above about 10,000 feet along the China 
coast and above 15,000 feet elsewhere, summer winds 
aloft become easterly (Figure 158). Figure 160 shows 
the winds at 10,000 feet in July. 

The transition seasons differ from the monsoon 
seasons (relative to winds aloft) in that these transi¬ 
tion seasons, with their variable surface winds near 
the equator, have wider equatorial belts of variable 
winds aloft. 

FLYING WEATHER OF SPECIAL AREAS 
Flying Weather from India to Malaya (Bay of Bengal) 

The Bay of Bengal, like all areas of the Southwest 
Pacific and Indian Ocean, feels the monsoon influ¬ 
ence. Winters are clear and dry with generally good 
flying weather; summers are wet and cloudy. Spring 
is a period of transition with increasing cloudiness 
and precipitation. Thunderstorms associated with 
“western disturbances” reach their maximum over 
the Bay of Bengal in the spring months (March to 
May). The more severe of these have gales of 25 to 
50 knots and are called “nor’westers”. Autumn brings 
a decrease in the summer cloudiness and precipitation, 
but at this season a second period of thunderstorm 
activity occurs and tropical cyclones attain their 
maximum. 

The Winter Monsoon Period. Temperature 
ranges from an average of 67 °F. at Calcutta to an 
average of 80 °F. at Singapore. At Calcutta, tempera¬ 
tures have dropped to as low as 44 °F. and soared to 
as high as 98 °F. Singapore has had extremes of 
66°F. and 94°F. 

Wind. Surface winds, usually light, prevail from 
the northeast. A transition zone, between surface 
winds and upper winds, slopes from an elevation of 
about 30,000 feet at the equator to about 5000 feet 
at Calcutta. Between latitudes i2°N. and i4°N. this 
transition zone is at about 20,000 feet. Above the 
transition zone the wind direction is westerly (south¬ 
west to northwest). Average wind velocity increases 
with altitude and with latitude. Thus over Calcutta 
winds at 10,000 feet have a velocity of about 25 
knots, while at 25,000 feet they average about 45 
knots. South of latitude 2o°N. upper wind velocities 
are light, averaging only 25 knots at 25,000 feet over 
Rangoon. 

Clouds. The cold air mass moving off the conti¬ 
nent becomes increasingly unstable and cloudiness 
increases to the south. Winter cloudiness averages 
three-tenths in the northern part of the Bay of Ben- 


162 



gal and six-tenths in the Strait of Malacca. Cloud 
types are dominantly cumulus, and, except in squalls 
and showers, you’ll usually find ceilings of 2000 feet 
or better. 

Precipitation. Rainfall increases from 0.5 inch a 
month at Calcutta to 9.0 inches a month at Singa¬ 
pore. The number of days with thunderstorms shows 
a similar increase to the south. Calcutta has an 
average of one storm each winter month, while 
Singapore has eleven in December alone. 

The Summer Monsoon Period. Temperature. In 
summer, Calcutta with an average of 84°F., is hotter 
than Singapore with average of 8i°F. The ther¬ 
mometer at Calcutta sometimes reaches extremes of 
io8°F. and 68 °F.; at Singapore, 95 0 F. and 69°F. 

Wind. Surface winds are prevailingly SW. at 
about 5 to 7 knots. Winds aloft maintain surface 
wind direction (SW.) up to about 20,000 feet. At 
10,000 feet the average velocity is 20 to 25 knots. 
Above 20,000 feet the winds are southeasterly at an 
average velocity of 15 to 20 knots. 

Clouds. Average cloudiness during the summer is 
greater (seven- to eight-tenths) over the Bay of Ben¬ 
gal and Calcutta than at Singapore (five- to six- 
tenths). The clouds are almost entirely cumulus, 
strato-cumulus and cumulo-nimbus, and perfectly 
clear skies are a rarity, especially in August. 

Low clouds are most frequent in morning hours. 
Ceilings of less than 1000 feet are rare, but ceilings 
of 2000 to 3000 feet occur about one-half of the 
time. 

Precipitation. Local squalls bring strong winds, 
torrential rains, lowered ceilings, and greatly re¬ 
stricted visibility. (Fig. 161). These squalls are small 
and of short duration. After the wind subsides, how¬ 
ever, heavy rain may continue for several hours. This 
is the wet season. Calcutta averages 12 inches of rain 
per month during this season; Singapore averages 
7 inches per month. 

The Transitional Period—Spring. Temperature. 
March to May is the hot season in India and over the 
Bay of Bengal. During this period, before the onset 
of the summer rains, daily temperatures soar highest. 

Wind. The transition from the season of winter 
monsoon (with its winds from the continent and 
dry weather) to the season of summer monsoon 
(with winds from the ocean and cloudy rainy 
weather) begins in March and continues through 
April and May. You’ll find variable and relatively 
weak surface winds during this transition period. 
Aloft, winds above 6000 feet (over the northern sec¬ 
tion of the Bay of Bengal) remain westerly, as in 


winter. At 10,000 feet the average velocities are 10 
knots above Rangoon and 20 knots above Calcutta. 
At 25,000 feet they are 22 knots above Rangoon and 
26 knots above Calcutta.* 

Clouds. Cloudiness and the frequency of low ceil¬ 
ings increase rapidly during this season, reaching a 
maximum when the summer monsoon “breaks”.** 

Precipitation. Precipitation increases gradually in 
April and May as a result of generally increased 
thunderstorm activity and the pre-monsoon (nor’- 
wester) storms which form in the Bay of Bengal. 

Visibility. Along the coasts, dust storms are a fre¬ 
quent cause of poor visibility during this hot season. 
They are modified nor’westers in which there is little 
or no rain. 

The Transitional Period—Autumn. Tempera¬ 
ture. Autumn is cooler and more comfortable than 
spring. 

Wind. The southwest monsoon of summer con¬ 
tinues dominant in September, especially in the lower 
levels, but the winds are much weaker and become 
more variable as the season advances. The decay of 
the summer monsoon begins in the upper levels and 
progresses downward to the surface and southward 
—that is, the maritime air from the South Indian 
Ocean gradually decreases in depth, retreating 
southward, and is replaced by continental air 
from the north. Winds aloft in September are 
southerly at all levels up to 20,000 feet. By Novem- 

* Only meager wind aloft reports are available for the south¬ 
ern area, but winds up to 20,000 feet may be expected to be 
northeasterly and easterly at moderate velocities. 

* * The summer monsoon comes suddenly—early in June it 
breaks across the Bay of Bengal onto the continent, bringing its 
deluge of rain and ending the spring transitional season. 



Fig. 161. A forced landing during a monsoon squall 
(U.S. Navy photo) 


163 



her, prevailing winds aloft above 10,000 feet have 
become northeast over the Bay of Bengal. 

Clouds. Clouds and low ceilings are at their maxi¬ 
mum summer values at the beginning of this season, 
but gradually decrease as the season progresses to 
values characteristic of winter. 

Precipitation. Dry weather prevails over the north¬ 
ern part of the Bay of Bengal during this season, 
but the southern area, Ceylon to Singapore, has its 
maximum rainfall for the year. A second maximum 
of thunderstorms occurs with the decay of the mon¬ 
soon circulation, but these are usually milder than 
the “nor’wester” storms of spring. 

Tropical cyclones. An average of about ten 
“cyclones” per year originate in the Bay of Bengal. 
Of these, an average of two a year reach full hurri¬ 
cane intensity. The following table shows the fre¬ 
quency of severe, moderate and mild cyclones for a 
36-year period. 


"CYCLONES" IN BAY OF BENGAL, 1877 TO 1912 

Average number of “ cyclones ” each month 


Month 

Severe 

Winds 65 knots 
or more 

Moderate 
Winds 48-65 
knots 

Mild 

Winds less than 
48 knots 

April. 

.14 

.03 

.03 

May. 

.28 

.19 

.11 

June. 

.19 

.44 

• 55 

July. 

.17 

.81 

.87 

August. 

.06 

■58 

.88 

September. 

.19 

.70 

1.05 

October. 

.39 

.62 

.44 

November. 

.50 

.36 

.17 

December. 

.22 

.11 

.14 

Average per year 

2.14 

3.84 

4.24 


The months of January, February and March are 
periods of settled weather over the Bay of Bengal 
and tropical cyclones are very rare during this period. 
Cyclonic activity begins in April and continues 
through December, reaching a maximum in July, 
August and September for the mild storms. Real 
hurricanes, relatively infrequent during the summer 
monsoon, reach a maximum in October and Novem¬ 
ber and a secondary maximum in May. Tracks of 
these storms are shown on Figure 154. 

Flying Weather from Australia to Malaya 

This area extends from the equator to i5°S. 
Latitude and from ioo°E. to i45°E. Longitude. Its 

164 



Fig. 162. Cumulus clouds over tropical island 
(U.S. Navy photo) 


climate is controlled primarily by westerly mon¬ 
soons* from December to March and southeast mon¬ 
soons from May to September, with April and 
October-November as transitional inter-monsoon 
periods. The mountainous topography of the islands 
exerts a marked influence on local weather. Leeward 
and windward sides of islands differ; and high pla¬ 
teaus differ from lowlands, while inclosed mountain 
valleys differ from lowlands exposed to winds. 

In these latitudes the weather varies greatly be¬ 
tween day and night. Winds, clouds, and precipita¬ 
tion have a marked diurnal regime, and temperature 
varies more from day to night than from season to 
season. “Night is the ‘winter’ of the tropics.” 

The direction of the monsoons, however, deter¬ 
mines the migrations of the intertropical front and 
the movement of air masses both at the surface and 
aloft. The monsoons are thus the dominant control 
over flying conditions within the area. We may con¬ 
sider four seasons: 

(1) The westerly monsoon season. 

(2) The April inter-monsoon season. 

(3) The southeast monsoon season. 

(4) The October-November inter-monsoon 
season. 


* The westerly monsoon is usually a diverted northeast wind 
from the Northern Hemisphere, but when the intertropical 
front is north of the area having westerly winds, the westerly 
wind consists of trade air from the South Indian Ocean that 
modifies the equatorial air and veers to south, southwest, and 
finally to west as it is drawn into the Australian low. 































Westerly Monsoon Season (December to 
March). This is the rainy season. The intertropical 
front fluctuates over the area, and from the two 
hemispheres equatorial air masses (moist and un¬ 
stable) converge and move westward toward the 
low pressure (doldrum) area over Australia. Weather 
may be summarized as follows. 

Temperature. The average is about 8o°F. Nights 
are generally about io°F. to i4°F. cooler than days. 

Wind. Over the open sea the northeast monsoon 
from the China Sea changes to north as it approaches 
the equator, and to northwest and west over the 
Java Sea. Southeast trades from the South Indian 
Ocean veer to southwest and west, and finally even 
to northwest as they are drawn into the Australian 
low. Thus, westerly surface winds predominate, in¬ 
terrupted occasionally by the intertropical front 
with its variable winds. 

Winds are usually light, averaging 5 to 10 knots. 
On occasion you will meet strong winds in this area, 
especially in mountain passes and in channels between 
islands. Even the strongest surface winds of the re¬ 
gion, however, seldom exceed 30 knots, except in 
gusts. 

Near coastlines the winds are modified by land- 
and-sea-breeze influence. For example, in the north¬ 
west monsoon air current, land-and-sea-breeze influ¬ 
ence causes a strong daytime sea breeze on the 
northern coast of Java and a strong nighttime land 
breeze on the southern coast. The land-and-sea- 
breeze effect may be noticeable several miles seaward 
from the coast. The sea breeze (day) influence ex¬ 
tends to 3000 feet aloft, but the land breeze (night) 
influence is shallow. 

Winds aloft. With increase in altitude the average 
wind velocity increases to about 13 knots at 1500 
feet, maintains that velocity to 10,000 feet and then 
decreases slightly. The wind direction becomes even 
more dominantly west with increased altitude up to 
15,000 feet. Here easterly winds are encountered, 
which become more and more common up to 20,000 
feet and above. 

Clouds. Average cloudiness for the season is five- 
to six-tenths of sky obscured. There is a more or less 
regular diurnal sequence of cloudiness. 

Over land, stratus or strato-cumulus clouds form 
at night, clearing after dawn; and cumuliform clouds 
develop during the afternoon. If cumulo-nimbus 
clouds develop, land cloudiness persists until the 
nighttime land breeze starts blowing and moves the 
cumulo-nimbus to the coast. 

Over sea, cumulus or cumulo-nimbus clouds form 
at night rather than by day, especially along coastlines. 


It is a good rule never to fly into a cloud over tropical 
seas at night. 

Above the coastal plain of Java the base of cumu¬ 
lus clouds ranges from 2000 to 3000 feet, with tops at 
10,000 feet or more. Above the seas, the ceilings 
range between 1500 and 3000 feet. On exposed 
mountain slopes, ceilings are much lower, often zero 
during strong winds. 

Precipitation. Distribution of rainfall is influenced 
by topography. Rain is heavier over land than over 
sea, and heaviest on windward mountain slopes. Low¬ 
lands average about 10 inches per month; mountain 
slopes average 25 or more inches. 

Character of rain, also, is affected by topography. 
Continuous rain often falls on mountain sides ex¬ 
posed to the monsoon, while over level land and sea, 
rain is usually of the shower type. Rain storms over 
the sea can generally be avoided because they are of 
small diameter and can be seen for great distances. 

Thunderstorms and turbulence. The frequency of 
thunderstorms varies greatly with the topography. A 
large number of thunderstorms originate on moun¬ 
tain slopes during the day, then build out over the 
plains (Figure 162-). Thunderstorms rarely cross high 
mountains. Instead, after nightfall, the land breeze 
carries them back to the coast, where they may rage 
on into the night. Over land, thunderstorms occur 
mostly in the afternoon or evening. At sea, though 
rare at this time of year, they occur mostly at night. 

Even without thunderstorms, turbulence may be 
encountered over land by day and near the coast at 
night. You’ll find it in cumulus clouds and monsoon 
squalls. 

Visibility. Fog is not frequent but does form occa¬ 
sionally over plateaus and lowlands on clear nights. 



" WILLY-WILLY " SWINGS LIKE A SOUTHPAW 


165 




It lifts soon after sunrise. Visibility is usually good 
except in heavy rainstorms. 

Tropical Cyclones. Willy-willies harass the seas 
between Timor and the coast of Australia. They are 
most frequent in January, but may strike during any 
month between November and May. Refer to table 
on page 13 and Figure 154. 

Southeast Monsoon Season (May to Septem¬ 
ber). Near Australia this is the dry season. Singa¬ 
pore never knew a dry season! 

Temperature. The thermometer averages about 
80 °F., the same as the temperature during the rest 
of the year. Nights are about i2°F. to 2o°F. cooler 
than days, the greatest nighttime cooling occurring 
when skies are clear. 

Wind. Surface winds over the open sea are mainly 
southeast. Near coastlines, however, the direction of 
the monsoon is modified by land and sea breezes. 
Over the. north coast of Java, for example, the sur¬ 
face winds may even be northerly by day under the 
influence of the sea breeze. The sea breeze generally 
begins about 1000 or 1100 and lasts till after sun¬ 
down. It may extend seaward 25 to 30 miles and 
aloft to 3000 feet. It is usually strongest at about 
650 feet. The land breeze is weaker and shallower 
than the sea breeze and extends only to about 400 
feet aloft. 

Winds are usually light, averaging between 10 
and 14 knots near the equator. Inland, over large 
islands, surface winds are light, and night calms are 
frequent over level country. 

Winds aloft. Easterly winds of less than 15 knots’ 
velocity prevail to 10,000 feet above northeast Aus¬ 
tralia and the Timor Sea, and winds aloft are variable 
nearer the equator. At elevations from 10,000 to 
20,000 feet the wind direction is variable over the 
entire area. Here, never rely on the winds aloft. 

Clouds. The southeastern section (near Australia) 
has few clouds, averaging two- to three-tenths. In 
the north and west cloudiness averages four- to six- 
tenths. The diurnal variation of cloud type and 
amount is similar to that of the westerly monsoon 
season. 

Ceilings are much the same as in the westerly mon¬ 
soon season. 

Precipitation. Over most of the area this is the 
dry season. Especially dry are seas and islands just 
northwest of Australia where the winds blow off the 
hot desert. Farther west the winds have obtained 
much moisture and bring considerable rain to south¬ 
ern windward coasts and slopes. Singapore remains 

166 



Fig. 163. Cumulus meeting alto-stratus at the inversion level 
between Manila and Macar from an altitude of 8000 feet 
(U.S. Weather Bureau photo) 


wet. Northern coasts (in the rain-shadow of moun¬ 
tains), though dryer, are subject to thunderstorms. 

Thunderstorms and turbulence. Near Australia, 
thunderstorms are uncommon even over land when 
the southeast monsoon is fully established, but at 
the beginning and the end of the season they cause 
afternoon showers. Over western Java and the islands 
near the equator, thunderstorms are frequent, ac¬ 
companying most of the afternoon showers even 
though this is the season of fewest thunderstorms. 
A place in western Java may expect five thunder¬ 
storms in August, its least thundery month. 

Visibility. Visibility is frequently restricted, over 
both the land and the sea, by dust haze carried 
from the interior of Australia. At times visibility will 
approach zero in che vicinity of Port Darwin. 

Inter-Monsoon Seasons (April and October- 
November). During these inter-monsoonal months, 
winds are usually light and variable, and afternoon 
showers are common. 

Temperature. Don’t expect any change. It’s always 
hot. 

Wind. Generally light and variable, the ten¬ 
dency is for southerly winds. Land and sea breezes 
are pronounced along the coasts. Where mountains 
approach the coast, the land-and-sea-breeze effect is 
enhanced. 

Inland, surface winds are light, and night calms are 
very frequent over level country. 

Strong southwest winds sometimes occur in No¬ 
vember (and December) in western Java and south¬ 
ern Sumatra, especially in the mountain passes, on 
exposed and bare plateaus, and on top of the moun¬ 
tains of Sumatra. 



Winds aloft. Don’t “count” on them. They’re light 
and variable during both transition seasons. 

Clouds. These months have the same regular 
diurnal variation of clouds as in the monsoon sea¬ 
sons. In general, cloudiness is decreasing in April and 
increasing in October and November. Over land, the 
base of the cumulus clouds is about 2000 to 3500 
feet, but in wet weather on exposed mountain slopes 
the cloud bases sink much lower during a strong 
wind. Over water the ceilings are about 1500 to 
3000 feet. 

Precipitation. Rain occurs principally in the form 
of heavy showers and is nearly as frequent as during 
the northwest monsoon. 

Thunderstorms and turbulence. Thunderstorms are 
frequent during these months. Near the equator, 
their greatest frequency of the entire year occurs 
during these inter-monsoon seasons. Over land they 
occur mostly during the day and over the seas mostly 
at night. Even when thunderstorms do not exist, 
severe turbulence may be encountered in the vicinity 
of cumulus clouds. 

Visibility. Visibility is usually good except for 
local fogs at night in valleys, over plateaus, and over 
swampy land. These fogs usually dissipate shortly 
after sunrise. 

Flying Weather of the Philippines and South China Sea 

Although dominated by monsoon influence, the 
climate of the Philippines and the South China Sea 
area is complicated by frontal activity—polar front in 
winter and intertropical front in summer. The air 
masses at the surface are modified polar continental 
(cP) and tropical maritime (mT) in winter, and 
equatorial (E) and tropical maritime (mT) in sum¬ 
mer. Local usage terms the cP air “norther” or “win¬ 
ter monsoon”, the mT air “trade”, and the E air 
“summer monsoon”. The flying weather of this sec¬ 
tion can best be described by noting the average posi¬ 
tion and characteristics of the air masses and of the 
fronts that separate them. 

Winter Monsoon Season (November to 
March). Air mass weather. The mT air that invades 
this area comes from the subtropical high of the 
Pacific. Along the northern border of the Philippines 
this air mass is about 8000 feet deep, but its thickness 
increases rapidly to the south. Refer to Figure 157. 
The top of the air mass is marked by an inversion 
level, above which westerly winds blow. This inver¬ 
sion level is quite significant to fliers. Cumulus clouds 
in the mT air build up, on the average, to 8000- 
12,000 feet—to the inversion level (Figure 163). 
Patches of stratus sometimes blow out in thin sheets at 


the inversion level. Also the air is hazy below the in¬ 
version level. In the region of the Philippines and 
South China Sea, mT air blows over a sea surface of 
marked uniformity—neither much warmer nor colder 
than that to which the air “is accustomed”. The mT 
air, therefore, neither boils up into thunderstorms (as 
over equatorial waters) nor condenses its moisture 
into fogs and low stratus (as over cold northern seas). 
It brings relatively fair weather to the Philippines 
and South China Sea. 

The cP air comes from wintery Asia; it is cold and 
dry. Even more shallotv than the mT air, it has an 
inversion level that may be as low as 4000 feet, above 
which westerlies blow. Cumulus clouds form in the 
northern air over the warm South China Sea—small, 
sharply-defined cumuli with cloud tops at the 4000 
to 6000 foot inversion level. 

Having a low humidity (because it is cold and 
dry), the cP picks up moisture rapidly as it warms 
in passing over the sea. It becomes more unstable in 
its surface layers than does the mT air. When it 
rises over mountains (as along the northeast coasts 
of the Philippines and the coast of Indo-China) the 
mountain slopes cloud and rains pour. Since the 
moisture is confined to surface layers, the air mass 
is washed out on crossing the mountains and brings 
fine weather to leeward locations. 

By the time the norther air has reached the south¬ 
ern Philippines it has modified and become quite 
similar to the trade air. On approaching the equator 
the air masses (both norther and mT) are modified 

WINTER SEASON 



Fig. 164. Block diagram showing air masses 
off the coast of China 


167 



WINTER SEASON 



Fig. 165. Block diagram showing air mass 
relationship in Philippine section 


to equatorial air and flying weather corresponds to 
conditions at Singapore described earlier on pp. 164- 
167. 

Along the northern border of the South China Sea 
the westerly air mass of the upper air plays an im¬ 
portant role in the weather of winter. This upper 
current is extremely persistent from October until 
April or May. The average height of the base of the 
westerlies is 13,000 feet in October, decreasing gradu¬ 
ally to 4000 feet in April, and subject to considerable 
variation from day to day. The reversal of wind di¬ 
rection between the surface current of norther air 
and the upper westerlies produces a definite wind 
shear at this level, accompanied by a marked tem¬ 
perature inversion and decrease in humidity. A very 
distinctive layer of strato-cumulus clouds develops at 
this level off the China coast (Figure 164). 

It is during the latter part of the winter season, and 
early spring, that fog persists at Hongkong and over 
adjacent coasts and mountains. The late-winter 
lowering of the westerlies coincides with formation 
of a low pressure trough in the Yangtze Valley and 
the weakening of the winter monsoon. Then dry and 
comparatively warm air pushes off the continent over 
the cold coastal ocean current flowing down the 
China coast. This results in advection fog. This fog 
as a rule does not extend more than 50 miles out to 
sea, but may extend up to 10,000 feet. It varies in 
density from thick haze to damp cloud. Frequently 
a patchy deck of strato-cumulus clouds develops in 

168 


friction layers close to the surface of the water. 
Visibility is usually best between this surface layer 
of clouds and the cloud layer at the inversion level 
(4000 to 6000 feet). 

Frontal weather. The polar front separating trade 
(mT) air and norther (cP) air usually extends from 
northern Indo-China northeastward between the 
Philippines and Japan. Since the two air masses have 
the same direction (northeast) and similar character¬ 
istics, no lows originate along the front. If, however, 
the trade air swerves into a southeasterly direction 
and meets norther air in the vicinity of Taiwan, mov¬ 
ing depressions form that intensify as they move 
northeastward off the coast of Japan. In the Philip¬ 
pine area the front usually appears as a row of cumu¬ 
lus, heavy cumulus, and cumulo-nimbus when the 
northers are surging, and as a row of strato-cumulus 
of some thickness when the trades push over rela¬ 
tively stagnant northers. Ceilings in the frontal zone 
are about 2500 feet and visibility is good unless it 
rains. 

It is of value to flight crews operating in this 
section to be able to locate frontal surfaces and pick 
up the shortest route across them. For example, a 
pilot flying from Guam to Manila starts out in trade 
air with good flying weather. Now, if he gets into 
the towering clouds of a frontal zone lying nearly 
parallel to his course (as it often is), he can save him¬ 
self a good deal of frontal flying by getting into the 
norther air as soon as possible. Once in this air he 
will have nearly ideal flying weather (Figure 165). 

The front can nearly always be negotiated in clear 
air below 10,000 feet by threading around the tops 
of the cumulus clouds. The main hazard is the occa¬ 
sional cumulos-nimbus and its attendant squalls. 
There is little danger in flying through cumulus 
clouds whose tops are below the icing level (in the 
latitudes of the Philippines the icing level is at 15,000 
to 20,000 feet; at Hong Kong, 8000 to 16,000 feet). 
Cumuli that penetrate the icing level, however, be¬ 
come cumulo-nimbus with dangerous turbulence. 
Hence, it is important to be able to see the top of all 
convective clouds to be entered. The hazard is great¬ 
est at night, for then cumulo-nimbus are hardest to 
see and most turbulent. When you’re flying at night 
and encounter patches of alto-cumulus or stratus, or 
light to moderate turbulence, regard it as a warning 
of a cumulo-nimbus cloud in the vicinity. The ab¬ 
sence of static in the radio is not assurance that there 
are no cumulo-nimbus, because on winter nights 
there is often no electrical activity in the cloud 
though there may be vicious up- and downdrafts. 
Do not enter heavy clouds at night! 



Summer Monsoon Season (April to October). 
Air mass 'weather. The equatorial (E) air mass may 
be encountered as far north as Hong Kong from 
April to August. It reaches the Philippine area in 
spring and lingers until October, or even November 
or December. The “opponent” air mass to the equa¬ 
torial in summer is usually the trade (mT) . Both air 
masses have traveled great distances over warm seas. 
As we should expect, their physical properties in 
surface layers are similar, yet they are in marked 
contrast as regards weather, for mT air brings “fair 
weather cumulus” skies, while E air brings heavy 
cumulus or cumulo-nimbus, often with showers. The 
real distinction lies in the fact that although both air 
masses are warm, moist and unstable at the sur¬ 
face, the mT air becomes dry and stable at a height 
of about 8000 feet (its inversion level). In mT, few 
cumuli extend above that level. The E air, in con¬ 


trast, is moist and unstable to great height. Very 
little initial lifting is sufficient to start almost un¬ 
limited ascent of saturated air. In the E air, then, 
cumulus or cumulo-nimbus are invariably present, 
particularly on windward coasts and along wind¬ 
ward slopes. These low cumuliform clouds are gen¬ 
erally detached, and alto-stratus or alto-cumulUs ap¬ 
pear through breaks in the lower clouds. The base 
of the low clouds is at an average height of 2000 
feet, and the tops may tower to great heights. 

As a rule, air mass rainfall is of shower type, 
(Figure 166). Prolonged rainfall is confined to 
frontal regions. 

Visibility. Visibility is variable. It is often excellent 
in light winds and moderate in a strong southwest 
monsoon, probably owing to the presence of sea salt 
nuclei carried into the air by spray. 



Fig. 166. Tropical showers (U.S. Navy photo) 


169 






Frontal weather. During late April or early May, 
the intertropical front hovers near the equator, and 
then it moves northward as summer advances. Dur¬ 
ing July, it extends from northern Indo-China 
through the channels between Taiwan and Luzon, 
and from there roughly southeast to near Guam and 
Yap in the Carolines. The SW. monsoon (E) air 
occupies all the area south of the front, while to the 
north of the front are the Northern Hemisphere 
trades (mT air) and occasional north (cP) air drawn 
south behind moving cyclones. As the summer pro¬ 
gresses, the SW. monsoon may extend as far north 
as Shanghai. When the NE. monsoon begins to 
re-establish itself in September, the SW. monsoon 
steadily retreats toward the equator, but in the 
Carolines it still can remain quite strong. By late 
November or early December, the intertropical front 
has receded to the equator or beyond. 


In the vicinity of the Philippines the intertropical 
front is sometimes in a north-south position, some¬ 
times east-west. When it lies in a north-south posi¬ 
tion over the South China Sea, it is very weak, be¬ 
cause the SW. monsoon (E) air to the west and the 
mT air to the east both move from warmer to cooler 
climates, which makes for stability. When, however, 
the intertropical front lies north-south along the 
Philippines, many thunderstorms develop along the 
front. This is due to the surface heating of the air 
over land and the clashing of the two air masses re¬ 
sulting from the added effect of the land and sea 
breezes. 

When the intertropical front takes an approximate 
east-west direction through the Philippines and the 
South China Sea, a very mild pressure trough re¬ 
sults. This situation usually forces the SW. monsoon 
(E air) aloft, and rain falls from alto-stratus clouds. 

If, as sometimes happens, especially in autumn, 
norther (cP) air feeds into the intertropical front, 
we then have three air masses converging on a point 
where the polar front intersects the intertropical 
front (Figure 167). Such a situation involving three 
air masses and three fronts (one for each pair of air 
masses) is called a “triple point”. Look out for 
typhoons under such conditions—especially when the 
intertropical front lies in an east-west position near 
the Carolines, or between Taiwan and Luzon. 

For discussion of typhoons see pp. 144-148. These 
storms are the greatest weather hazard of the area. 



170 


















JAPAN AND NEIGHBORING SEAS 


Your principal target and ultimate goal in the Pacific lies off the eastern coast of Asia. Remember that 
the enemy knows this area far better than you—he will be able to use to advantage every character¬ 
istic of weather and terrain. He is familiar with the region and, in addition, has a special advantage 
relative to forecasting: the weather of the North Pacific moves from west to east. This gives the Japs 
advance notice of weather on our side of the Pacific, whereas we get no information of weather on theirs. 
You cannot learn the nature of this area too well—you must never cease inquiring about the climate and 
weather of Japan and surrounding seas, analyzing the facts you learn and fitting them into your Dicture 
of the general climatic pattern. 


In this final chapter we shall use our understand¬ 
ing of the climatic pattern of the Asiatic Pacific, and 
piece together the pictures of its northern and trop¬ 
ical sections into a co-ordinated panorama, in order 
to get a clear picture of the climatic setting of the 
Nips. We shall analyze the flying conditions around 
and over our target area—Japan. 

THE CLIMATIC CONTROLS 
Land and Water; and Latitude 

Located as it is between the greatest land mass 
and the largest ocean of the world, Japan’s wind sys¬ 
tem feels the influence of the seasonal pressure 
changes taking place between these areas of contrast. 
This story of how temperature controls pressure, and 
pressure controls air mass movement and wind, is too 
familiar to you to require explanation here. Figures 
11 and 12 show average air mass movements of this 
area. 

Bear in mind that we are dealing with an eastern 
coast—definitely a leeward coast during the winter 
season, when westerlies dominate (Figure n). The 
location should be compared with the Atlantic coast 
of the United States, not with our Pacific coast. 
Thus, Karafuto, Hokkaido and northern Honshu 
have seasonal conditions somewhat similar to Mari¬ 
time Canada and New England—note the similarity 
in latitude. Southern Honshu, Shikoku and Kyushu 
compare somewhat with our southeastern states; and 
Taiwan (Formosa) is situated in the same latitude as 
Habana, Cuba. The climate of the Japanese Islands, 


however, does not exactly parallel the climate of 
the American Atlantic coast. The differences result 
from (i) the strong monsoonal wind system men¬ 
tioned earlier, (2) the mountainous topography of 
Japan, and (3) the ocean currents bathing both east 
and west shores of these islands. 

Land and water distribution, and latitude, deter¬ 
mine the general temperatures and the air mass move¬ 
ments. There is need, however, of noting topogra¬ 
phic features of the Japanese Islands and the ocean 
currents of surrounding, seas in order to see where 
and how the air masses develop the characteristic 
weather and climate of the different sections of 
Japan. 

Topography 

Look at the map (Figure 168). Japan consists of a 
series of rugged islands fringing the eastern coast of 
Asia. This archipelago is literally the crest of a great 
range of mountains rising from the ocean floor. The 
surface of the islands is rugged—mountain peaks 
reach a height ranging from 5000 to 13,000 feet. 
Fuji, southwest of Tokyo, is 12,461 feet and Niitaka, 
on Taiwan, 13,035 feet above sea level. The moun¬ 
tainous character of the islands has an extremely im¬ 
portant influence on flying conditions over this area. 

Ocean Currents 

Prominent among the climatic controls in the vi¬ 
cinity of Japan are the ocean currents (Figure 169). 
The Sea of Okhotsk has been called the “ice cellar” 
of the Pacific Ocean. It yields more ice and cold 
water than the Bering. Sea. From this Sea of Okhotsk 


171 



172 


Fig. 168 . Locational and physical map of Japan and neighboring seas 



















a cold current flows out between the Chishima 
(Kuril) Islands. Cold water from this current unites 
with the cold waters of the Kamchatka current from 
the Bering Sea, and this cold current flows south- 
westward along the east coast of Honshu to approxi¬ 
mately 38 °N. Here, especially in winter and spring, 
it meets the warm Japan Current which flows north¬ 
ward along the southeast coast of Japan. This meet¬ 
ing of cold and warm currents is comparable to the 
meeting of the Labrador Current and Gulf Stream 
southeast of Newfoundland. Like its counterpart in 
the North Atlantic, this area is a potent source of 
fog, chiefly in spring and summer when much warm 
moist air blows northward over the cold water. In 
summer, over 50% of the days are foggy in this area 
as far south as 40°N. 

Another cold current, the Liman, flows out of the 
Sea of Okhotsk through the Tartar Strait between 
Karafuto and the Siberian coast and meets the Tsu- 
sima warm current which flows north along the west 
coast of Honshu and Hokkaido. In late spring and 
summer, when warm south winds sweep over the 
cold water, persistently-enduring sea fogs frequently 
cloak the Sea of Japan. Even more persistent fogs 
cloak the Sea of Okhotsk. 

WEATHER IN THE DIFFERING AIR MASSES 
AND FRONTS OVER JAPAN AND 
NEIGHBORING SEAS 

Air Mass Weather 

The air masses that affect Japan include the polar 
continental (cP) from Asia, the polar maritime 
(mP) that sometimes circles back from the North 
Pacific, the tropical continental (cT) from Asia, 
and the tropical maritime (mT) from the subtrop- 
! ical high of the Pacific. The characteristics of these 
air masses have already been described. We are now 
concerned largely with their modification within the 
Japanese area, and with their frontal activity. 

The modification of the cP air mass as it moves 
over Japan during the winter accounts for the nor¬ 
mal weather of the winter season. Beginning in Sep¬ 
tember in the northern islands and as late as Novem¬ 
ber at Latitude 30°N., northwesterly winds carry 
cP air over the Japanese Islands. Cold and dry in its 
source region, this air warms on crossing the Sea of 
fapan, picks up moisture, and becomes unstable in 
ts lower layers. Considerable cloudiness results, but 
ceilings over the Sea of Japan are normally moder¬ 
ately high (3000 feet) and visibility is usually good. 
Occasional dust storms over the Taiwan Strait and 
'fellow Sea, however, sometimes reduce visibility to 
tearly zero in these areas. 



As the unstable cP air rises over hills along the 
west coast of Japan, moderate to heavy rain or snow 
results, with consequent lowering of ceiling and visi¬ 
bility. This precipitation occasionally starts well out 
over the Sea of Japan. Where ascent of air is steep¬ 
ened by the mountains that form a climatic barrier 
along the entire length of Japan, the cloud deck 
thickens, precipitation becomes more intense, and 
ceilings and visibility approach zero. Icing occa¬ 
sionally may be severe in heavy cloud decks 
(Figure 170). 

When the cP air mass has passed over the moun¬ 
tains, it has lost most of its moisture. Descending to 
the east and south coasts of Japan, it produces good 
flying weather. Cloudiness is at a minimum in the 
east and south coast section during this winter sea¬ 
son, visibility is generally good, and rainfall is at a 
minimum. 



Fig. 170. Modification of cP air over Japan in winter 


173 




























Fig. 171. Modification of mT air over Japan in summer 

To the southwest of Japan (over the Yellow Sea 
and in the Yangtze Valley of China) and frequently 
to the south of Japan (over the seas near the Philip¬ 
pine Islands), the cP air meets currents of mP or mT 
air. Active fronts form. Extratropical cyclones de¬ 
velop along these fronts, travel northeastward, and 
may pass over or near southeastern Japan, interrupt¬ 
ing the normally good winter flying weather of that 
section. Precipitation falls on an average of about 
10 days per month during the winter on Japan’s 
southeast coast, as compared with about 25 days per 
month on the west coast. Often it doesn’t rain on the 
southeast coast for periods of two or more weeks. 
Icing is occasionally a hazard, but is reduced by the 
low average cloudiness. 

As the Asiatic high begins to wane in spring, a 
transition to summer monsoon begins to take place, 
beginning in April in southern Japan and as late as 
June in the north. On the west coasts, flying condi¬ 
tions grow steadily better—there is less precipitation 
and fewer low ceilings. 

On the south and east coasts, however, spring 
brings a change for the worse in flying conditions. 
The southerly winds bring moist mT air into the 
area. This air, meeting with occasional outbursts of 
cP air from the continent, develops vigorous cyclonic 
activity along the southeast coast. Frequent precipita¬ 
tion, low ceilings, poor visibility, and occasional 
severe icing conditions pester pilots. 

By June the summer monsoon is established over 
all of Japan. mT air from the subtropical high now 
dominates the area, but southern Japan also receives 
occasional invasions of equatorial (E) air. 

The E air over southern Japan, as over the Philip¬ 
pines, is moist and unstable to a great height. Tall 
cumuli build up in this air when it moves over the 
warm Japan Current and the summer-warm islands. 
When equatorial air moves far north along the 
Japanese coast it becomes modified and forms a deep 
persistent fog. 

Remember that equatorial air is comparatively 
rare in all except the southernmost parts of the 
Japanese islands; mT air dominates the summer scene 
in Japan. mT is less moist than equatorial air, and has 

174 


an inversion level at about 6000 feet.* While surface 
winds are southerly and southeasterly during sum¬ 
mer, winds aloft, above the inversion level, become 
westerly. Over seas south of Japan the mT air may 
have low cumuli, with tops at the inversion level. 
When it flows northward over colder waters, how¬ 
ever, this air becomes stable and develops stratiform 
clouds that drip drizzles. When it reaches far north, 
fogs cloak the coasts and the ocean. 

Lifting of the mT air along the coast results in 
widespread cloudiness with light rain and drizzle. 
On being forced to ascend over the mountains, the 
mT air displays properties of instability, producing 
towering cumulus clouds (Fig. 171). Clouds pile up 
along the mountain range, frequently extending from 
near the surface to well above the highest peaks, so 
that the mountains are obscured to a pilot approach¬ 
ing Japan from the south or east. Due to the high 
moisture content of the air, the mountain clouds of 
the southeastern coast during summer are more ex¬ 
tensive than the mountain clouds of the west coast 
during winter. 

As the air mass descends on the west side of the 
mountains and sweeps beyond the coast across the 
Sea of Japan, it is adiabatically heated and absorbs 
(rather than condenses) moisture. In western Japan, 


* The inversion level, above which the wind is westerly, in¬ 
creases in height from north to south. It is 8000 feet or higher 
over the Philippines. This inversion level also increases in height 
in summer (Figure 158). 



JAPAN HAS WEATHER MUCH LIKE THAT Of 
OUR OWN EAST CO A S T 























then, air mass flying conditions are much better in 
summer than in winter. 

Except where clouds hide mountain slopes and 
where fogs form over cool seas and northerly coasts, 
summer ceilings are normally high (2000 to 4000 
feet) and summer visibility is fair. Haze, however, 
increases with the season, becoming worst in August. 
Icing at normal flying levels is rare during the sum¬ 
mer period. 

Frontal activity over Japan increases in June and 
July, and thunderstorms are frequent in late summer. 

Frontal Weather 

Polar Front. In winter when high pressure pre¬ 
vails over Asia, the polar front is crowded far south¬ 
ward. It normally takes a general NE.-SW. direction, 
extending from the South China Sea along the line 
of the Ryukyu Islands to east of Japan. During a 
strong outburst of cP air from Asia, the front moves 
southeastward over the Philippines and over the 
Pacific as far as Guam. Cyclones develop along this 
front and move northeastward across the North 
Pacific to the Aleutian low. Generally, in winter, 
this polar front lies so far east that its extratropical 
cyclones do not influence the flying weather of 
Japan. On about 10 days per month in winter, how¬ 
ever, polar front activity brings precipitation to 
southeastern Japan. 

I Owing to the strong Asiatic high with its out¬ 
flowing winds, frontal activity is not very prevalent 
over Asia during the winter months. However, after 
a great surge of cP air has pushed out beyond the 
continental borders, a new outburst starts from 
Siberia. Each such new outburst forms a front that 
advances southeastward across China. Shallow de¬ 
pressions develop along these fronts and yield light 
precipitation in China, Korea and Manchuria. When 
these storms reach the coast, they deepen and bring 
heavy precipitation, usually in the form of snow, 
over northern Japan. 

In spring, as the Asiatic high weakens, the aver¬ 
age position of the polar front shifts northwest¬ 
ward and often lies along the south coast of China 
and just off the south and east coasts of Japan. 
The frequency of cyclonic storms at this season is 
rather high—averaging 10 per month in March, April, 
and May. These storms present no flying problems 
differing from those of similar storms in the United 
States, except that precipitation is usually more wide¬ 
spread and ceilings are lower, due to the greater 
availability of water vapor. Icing conditions are 
sometimes severe. 



Fig. 172. Weather during the first raid on Tokyo 


Fig. 172 shows how a knowledge of the movement 
of cyclonic “families” has already been used in 
carrying out an attack on Japan. Unexpectedly rapid 
occlusion of the storm exposed our position to enemy 
craft and forced a launching of the planes 800 miles 
offshore into a headwind. The planes found shelter, 
after the raid, in a second storm over China. 

From March to June (while the polar front is 
retreating northwestward) and again in October and 
November (when strong cP outbursts surge anew), 
extratropical cyclones move out of Siberia and 
North China on an average of 2 or 3 monthly. Most 
of these storms cross the Sea of Japan north of Korea 
and proceed northeastward near or over Hokkaido. 
They are often accompanied by vigorous secondary 
cold fronts which move toward the east nearly along 
the 40th parallel in Japanese waters. The northerly 
and westerly winds behind these storms may have 
a velocity of 40 to 50 knots over the Sea of Japan, 
northern Honshu, Hokkaido and the Kurils. 

By midsummer the polar front has shifted north 
over Asia to at least the 50th parallel. This summer 
front is weak and, lying far north, has little effect on 
the climate of southern Japan. It does, however, 
bring disturbances to Karafuto and Hokkaido. 

The Coastal Front. Weather maps of summer 
generally show two fronts in Asia: one extending 
east-west approximately along the 50th parallel (the 
midsummer polar front referred to above), and the 
other paralleling the coast of Asia from northern 
Indo-China to Manchuria. Often these two fronts 
unite over Hokkaido or Karafuto and extend as a 
single (polar) front across the northern border of 
the Pacific Ocean. 


175 








The coastal front has a pronounced influence on 
the summer weather of Japan. Its outstanding feature 
is that it remains nearly stationary for long periods. 
During early summer, mid-June to mid-July, it 
usually extends northeastward from the Yangtze 
Valley of China across the East China Sea and thence 
along the Japanese Islands. During this period, mild 
extratropical cyclones along the coastal front be¬ 
come blocked in their forward movement, almost 
stagnating over southern and eastern Japan. (A 
strong subtropical high lies east of Japan and re¬ 
tards the forward movement of the cyclones.) These 
stagnating cyclones discharge continuous rainfall for 
days before they move on seaward. They greatly 
augment the early-summer precipitation and cloudi¬ 
ness of the southeast coast of Japan and interrupt the 
generally fair weather of the west coast and the Sea 
of Japan. This early-summer rainy period is known 
by the Japs as the Bai-u or “plum rains” because it 
coincides with the plum ripening time. It is a most 
disagreeable period, owing to the overcast skies, light 
winds, high temperature and humidity. 

Over the East China Sea and Ryukyu Islands the 
early-summer rainy period begins in May. There is 



one important difference between the rains here and 
those in southern and eastern Japan. In the East 
China Sea the rains occur as frequent showers rather 
than more or less steady light rains and drizzles. The 
average visibility is therefore much better in the East 
China Sea than on the south and east coasts of Japan. 

By late July the coastal front has moved inland. 
This increases the late summer rainfall of North 
China and permits a lessening of rain in Japan. 

WEATHER HAZARDS AND HELPS TO AVIATION 
THE GENERAL PATTERN 

Winds 

Surface Winds. Throughout the extent of moun¬ 
tainous Japan, land configuration and varying sea 
exposures are important in determining surface wind 
directions. Nevertheless, though locally disrupted or 
altered by these topographic influences, a general 
pattern of wind direction exists. Including its south¬ 
ern and northern chains of small islands, Japan 
stretches from tropical to subarctic climatic zones. 
The general wind pattern over this long archipelago 
varies with latitude and with season (Figure 173). 

Over southern and middle Japan, the surface 
winds are primarily monsoonal, blowing from the 



Fig. 173. Winds and cloudiness, January and July 


176 
















Asiatic high of winter and toward the Asiatic low 
of summer. In the extreme north, the winds are af¬ 
fected in winter by the great Aleutian low, which 
varies between the Gulf of Alaska and Kamchatka. 
In summer, when this high-latitude oceanic low fills 
in and pressure flattens, the winds of the northern 
area become weak and variable. 

In general, during 'winter (late September to early 
April), the Ryukyu Islands have northeasterly 
(north to east) winds, interrupted only by occa¬ 
sional extratropical cyclones. Over Japan Proper, 
the winter wind blows from the northwest; over 
Hokkaido and the Kurils, the winds are westerly 
(southwesterly to northwesterly); and in Karafuto 
and Sakhalin, winds blow from west or southwest. 
The mean velocity of the winter winds in Japan 
Proper and neighboring waters is 8 to 15 knots. 
Especially strong gales may develop at the rear of 
extratropical cyclones. Even as far south as the 
Ryukyu Islands, winds sometimes reach gale pro¬ 
portions. In the southern Japan Sea, ships record 
gales in about 5% of the observations from Novem¬ 
ber to February; and velocities have been known to 
rise as high as 50 knots. Off the east coast of Honshu, 
gale frequency is slightly higher and velocities of 55 
knots have been encountered. Gales are strongest 
and most frequent in the waters surrounding the 
northern islands (Hokkaido, Karafuto, and the 
! Kurils, Figure 97). From December to March, 10% 
to 17% of ships’ observations record gales in 
these waters, and maximum velocities may reach 60 
knots. These gales, often accompanied by long-con¬ 
tinued blinding snow, may prevent air operations for 
a day or more. 

In spring, the winds come increasingly from 
southerly to easterly directions; and by summer, 
winds from south and east attain their most pro¬ 
nounced stage of development. These summer winds 
are rarely as strong and steady as the north to west 
winds of the cold season. Locally, there are many 
variations in the prevailing winds. Japanese ob¬ 
servers have remarked that directions differ at all 
stations. Thus, for most of the islands (aside from 
the Ryukyu, where the monsoons have a high degree 
of development), Japan’s summer monsoons show 
much unsteadiness. The average velocity of summer 
winds is much less than that of winter monsoons; 


but southern areas of Japan have typhoons in July, 
August, September, and October—and typhoons 
aren’t average. 

Winds Aloft. Above 10,000 feet, strong westerly 
winds prevail throughout the year. In levels below 
10,000 feet the winds aloft undergo seasonal changes 
in direction not greatly different from the seasonal 
direction changes of surface winds. 

In winter, winds aloft in levels below 10,000 feet 
range from west to north in southern Japan and 
from west to southwest in the northern islands. (In 
the vicinity of Tokyo, average velocities are from 
15 to 30 knots in the lower levels, increasing to 25 to 
35 knots at 10,000 feet.) During the spring, southerly 
winds (in lower levels) increase in frequency as the 
season progresses; and during the summer, southerly 
and southwesterly winds aloft prevail at lower levels. 
Autumn is another transition period with the south¬ 
westerly winds aloft still prevailing in lower levels 
during the early part of the season. The northerly 
component increases progressively during October, 
until northerly and northwesterly winds are com¬ 
mon at all levels up to 10,000 feet. 

Typhoons 

Typhoons are the tropical cyclones of the Far 
East. Like the West Indian hurricanes, the typhoon 
has a very low-pressure center with a counterclock¬ 
wise circulation of winds of great intensity. The 
diameter of the wind system varies. It may be very 
small, even less than 50 to 100 miles, or it may be 
300 to 500 or more miles. In any case, it is likely to 
increase in size as it moves into high latitudes. 

The average number of typhoons for the entire 
Asiatic Pacific is about 20 per year, though not all 
of these attain excessive wind velocities. They are 
most frequent in the late summer and fall, and least 
frequent in midwinter. The table below indicates 
the average number for each month. 

From January to April very few typhoons enter 
the Japanese area. In May, Taiwan and the East 
China Sea are likely to experience a typhoon, and 
there is record of one crossing Kyushu and the Japan 
Sea in this month. In June, the storms often move 
over Taiwan and the Ryukyu Islands to the south¬ 
east coast of Honshu. July, August, September, and 


TYPHOONS OF THE ASIATIC PACIFIC 


Month 

Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

Year 

Average 

Frequency 

4 

.2 

.3 

.4 

.7 

1.0 

3.2 

4.2 

3.6 

3.2 

1.7 

1.2 

20.1 


177 





















October are the height of the typhoon season, and 
all of southern Japanese waters and coasts are likely 
to be visited by intense storms. By October, the East 
China Sea and the Sea of Japan are free of typhoons, 
but the south and east coasts of Japan are still 
strongly affected and afflicted. By November, the 
typhoon season is over in Japan. Only an occasional 
November storm strikes eastern Honshu, but the 
waters to the southeast and in the Philippine area 
may still experience a few typhoons (Figure 174). 

You know the indications of an approaching tropi¬ 
cal cyclone. If you use the facts you have learned 
about avoiding these severe storms, you should “live 
to be a gray-haired wonder” so far as typhoons are 
concerned. 

Thunderstorms 

The heart of the thunderstorm belt of the Pacific 
lies south of the extreme southwestern part of the 
region under discussion, extending from the Philip¬ 
pines southeastward to Indo-China. The frequency 
of storms decreases from south to north along the 
Japanese Archipelago. The thunderstorm season lasts 
from early spring to late autumn, but thunder may 
occur during passage of winter cold fronts. In the 
East China Sea and off southern Japan, 4% to 6% of 
ships’ observations show electrical disturbances from 
May to September. In central Japan and the Sea 
of Japan, late spring, early autumn and midwinter 
show greatest expectancy—that is, before and after 
the “plum rains” and during the burst of the cold 
monsoon and its polar front. Northern Japan experi¬ 
ences 1 or 2 thunderstorms a month from June to 
August, 2 to 4 a month from September to Novem¬ 
ber, few in winter and spring. 

Waterspouts may also occur during the summer 
season (Figure 175). 

Cloudiness and Ceilings 

Clouds can be either an aid or a hazard to air 
operation depending on the type of cloud and the 
extent of cloudiness, and upon the mission. Average 
figures give only a vague impression of actual condi¬ 
tions, yet they do serve to delimit certain areas as 
unfavorable. 

The islands and seas both to the north and south 
have a greater average cloudiness than do the waters 
surrounding Japan Proper. 

The Kuril Islands are cloudy throughout the year. 
It is relatively seldom that the mountain tops are 
visible from the surface, for the prevailing ceiling 
is below 1000 feet. In summer, when southerly 
winds predominate, the warmer air cools as it moves 
northward. This results in low cloud layers that, on 

178 


the average, cover eight-tenths of the sky. Clear days 
are practically nonexistent in summer, but short pe¬ 
riods of clear weather of from 6 to 8 hours dura¬ 
tion may occur. In autumn and winter, cloud ceil¬ 
ings improve with a decrease in the amount of cloudi¬ 
ness to a minimum of five- to seven-tenths. Cumuli- 
form clouds prevail in winter, and the bases of the 
cumuli are generally between 1000 and 1500 feet. 
Clear periods, lasting from 6 to 10 hours, are more 
frequent than in summer. 

Northern Honshu, Hokkaido, and Karafuto par¬ 
take of the summer cloudiness on their east coasts 
and of the winter cloudy period on their west 
coasts, with a lessening of cloudiness in spring and 
autumn. 

Winter is the cloudier period on all coasts exposed 
to the NW. monsoon winds that blow off the Sea 
of Japan. The clouds, dominantly strato-cumulus, 
are normally fairly high over the sea (2000 to 4000 
feet), but they grow progressively lower toward the 
Japanese coast. Ceilings frequently lower to 1000 
feet or less by the time the coast is reached, and they 
lower nearly to zero as the damp sea air condenses 
its moisture against the mountain slopes. On the con¬ 
tinental coast of the Sea of Japan, the average cloud 
cover of winter is only five-tenths. In western Japan, 
the average winter cloud cover is eight- to nine- 
tenths, with frequent overcast days. The daily period 



Fig. 174. Typhoon tracks over Japan 
























of maximum cloudiness is, on the average, shortly be¬ 
fore 1200, and the maximum cloudiness occurs after 
midnight. Cloudiness along this west coast lessens 
in spring, increases somewhat in early summer (plum 
rain period), and from August to early October 
again decreases in advance of the heaviest cloudiness 
of winter. Ceilings in summer are higher than in 
winter, and there are frequently several consecutive 
days of generally fair weather. 

The East China Sea and its enclosing Ryukyu 
Islands likewise have a maximum of cloudiness in 
the cooler months. The average cloudiness over this 
sea and island chain ranges between seven- and eight- 
tenths sky cover except from July through October 
when it decreases to five- and six-tenths over the 
water areas. The number of cloudy days ranges from 
15 to 20 per month in the cloudy period, to 10 to 15 
per month from July through October. Diurnal 
variation is not very pronounced, but there is a 
tendency toward two maxima: a period of greatest 
cloudiness between 1400 and 1600 and a secondary 
period between 0600 and 0800. Although no statisti¬ 
cal data are available on frequency of low ceilings in 
the East China Sea, it is safe to assume that the an¬ 
nual frequency of extremely low ceilings (less than 
1000 feet) is less in the East China Sea than in the 
Sea of Japan, because the East China Sea has no 
cold current and little orographic uplift along the 
coasts. The frequency of moderately low ceilings 
(1000 to 2000 feet) is probably greater in the East 
China Sea than in the Sea of Japan because of the 
greater frequency of cyclonic disturbances, especially 
in spring. 

East of Taiwan and the Ryukyu, the area of sub¬ 
tropical high pressures has low cloudiness and good 
flying conditions throughout the year, but north¬ 
ward toward the east coast of Japan Proper, summer 
cloudiness increases. You are probably most inter¬ 
ested in the area from Osaka to Tokyo. Here the av¬ 
erage cloudiness ranges from four- to five-tenths in 
winter to about eight-tenths in June. (We will let 
you explain why this differs from the west coast.) 
Overcast skies with light rain or snow occasionally 
interrupt the fair winter weather. (Again you know 
the cause.) Winter cloudiness with rain or snow is 
more frequent at Osaka than at Tokyo. This differ¬ 
ence is brought about by west winds that reach 
Osaka from the Inland Sea having avoided the moun¬ 
tains over which the northwesterlies pass. Neverthe¬ 
less, fair weather is the rule along all the southeast 
coast of Japan. 

In southeastern Japan, the period from February 
to May constitutes a long transition season between 
the drier winter and the wetter summer. During 



Fig. 175. Waterspout in Yangtze River, from USS Pittsburgh 


these months the flying conditions are variable with 
increasing cloudiness and precipitation, and with a 
growing frequency of low ceilings and visibility; 
that is, flying conditions are still good in February 
and March but grow worse until the “plum rains” 
of early summer set in. 

The mean cloudiness increases to about seven- 
tenths at Osaka and eight-tenths at Tokyo during the 
rainy summer season, and ceilings and visibilities are 
continuously very poor. 

At Tokyo, cloudiness reaches a maximum between 
0600 and 0700. This corresponds to the period of 
day during which stability due to night cooling 
is at its height, at which time low stratus clouds and 
fogs are likely to form and produce low ceilings and 
low visibilities. A secondary cloudy period from 
1400 to 1600 corresponds to the period when con¬ 
vection is at its height and cumulus clouds are most 
likely to form. The daily period of best flying 
weather comes between 2000 and 2200. 

Visibility 

The principal causes of low visibility are (1) the 
persistent summer fogs in northern waters and (2) 
precipitation. Locally, haze also limits visibility. 


179 





Haze. In spring and summer, haze is frequent in 
the Sea of Japan and East China Sea. It results, in 
part, from dust brought by winds from the deserts 
of Mongolia. In spite of the occasional dust storms, 
however, the southwestern part of the Sea of Japan 
has many days with exceptional visibility (an annual 
average of n% as compared with 4% to 6% in 
Pacific waters at this latitude). Haze is rather char¬ 
acteristic of mT air, partially due to fine particles 
of salt held in suspension. Thus, hazy atmosphere is 
prevalent in spring, summer, and fall, especially just 
prior to the passage of typhoons. 

Precipitation. Whether caused by frontal activity 
or by air mass activity, precipitation reduces vis¬ 
ibility. Over southern Japan and the warm sur¬ 
rounding seas, precipitation often falls as showers 
—showers which only momentarily obstruct vis¬ 
ibility. When air masses rise over mountains, how¬ 
ever (as on the Japanese west coast in winter and 
east coast in summer), and when warm air masses 
move far north over cold seas (as along the east 
coast and along the Kurils in summer), steady pre¬ 
cipitation may reduce visibility for long periods. 

Southeastern Japan and the Ryukyu region has 
two seasons of maximum precipitation. One, in spring 
and early summer, is the season of the “plum rains”, 
resulting from frontal disturbances that may linger 
for days over southern Japan. The other, in August, 
is the season of the heavy and widespread rains 
associated with typhoons. The air flow from the 
south brings much moisture throughout the entire 
summer, however; and even winter, the driest period, 
has some precipitation caused by activity of that 
polar front which normally lies east of Japan during 
this season. (Ships’ observations show a winter maxi¬ 
mum over the ocean east of Japan.) 

More precipitation falls on the west central coast 
of Honshu than anywhere else in Japan. The annual 
average in this region is 100 inches, and, more 
significant, 200 days a year have precipitation. 
Winters are wettest, because in that season the 
dominant air mass movement is from the Sea of 
Japan onto the west coast. Much of the winter 
precipitation of western and northern Japan comes in 
the form of snow. Don’t try to remember the figures, 
but snow occurs on an average of 30 days each 
winter in southwestern Honshu, increasing to 126 
days in Hokkaido and 144 days on the Kuril 
Islands. 

The Kuril Islands, Hokkaido, and northern Honshu 
have many snow or rain showers in winter and 
much steady rain or drizzle in summer. In the Kurils, 

180 


light, dry snow may be expected from November 
to April; and even Tokyo, on the east central coast 
of Honshu, has 14 snowy days in winter. In summer, 
when mT air moves far north, low stratus clouds 
form and drizzles fall. 

Fog. Fog seldom forms over the ocean south of 
Japan. The summer fog belt begins north of the 35th 
parallel. Between 35°N. and 4o°N. over the southern 
part of the Sea of Japan, fog occasionally develops 
from March to September; but even near the Honshu 
coast, where this fog is most frequent, it occurs in 
the foggiest month of June in only 5% of the obser¬ 
vations. The heaviest fog in these latitudes forms to 
the westward in the Korean waters of the Yellow 
Sea, where 15% to 20% of the observations report 
fog in June and July. 

Along the approaches to Vladivostok in the north¬ 
western part of the Japan Sea (40°N. to 45 °N.), fog 
is recorded in 30% to 40% of the observations during 
the heavily fogged months of June and July. The 
frequency of fog increases toward the north. Thus, 
for June and July, the percentage of ships’ observa¬ 
tions recording fog is as follows: along the north¬ 
west coast of Honshu, 21%; in the waters about 
Hokkaido, 25%; over the Sea of Okhotsk between 
Karafuto and the Kuril Islands, 40%. 

Eastern coasts are foggier than western. Here the 
cold Kamchatka Current and the warm Japanese 
Current meet to form the very heart of the fog 
belt. On the east side, as on the west side, fog in¬ 
creases from south to north. Off the coast of Honshu 
at 35 °N., 7% of the observations record fog in 
July, while east of Hokkaido and the Kuril Islands, 
40% to 50% of the July observations show it. The 
fog period is April to September; the percentage is 
less than 5 % during winter months, even in the Kurils. 
The distribution of coastal “fog holes” is quite irregu¬ 
lar, but the east coasts have more than the west, and 
all parts are consistently foggiest during summer. 

Far-reaching fogs at times extend for a great dis¬ 
tance seaward into the western part of the Pacific. 
Fogs here usually set in with easterly or southerly 
winds, and clear with a shift to westerly. While 
many of these fogs may break at noon, they have 
been known to last for a week at a time—especially 
during July and August. 

Along the volcanic chain of Kuril Islands extend¬ 
ing from Hokkaido to Kamchatka, fog is a real 
hazard. Forty per cent of ships’ observations indicate 
fog in eastern Kuril waters, but the leeward western 
side of the Kurils is much less foggy. Shana, the only 


station giving continuous record, shows an average 
of 46 days of fog annually, of which 44 are from 
May to September, 12 in July alone. This, it may be 
emphasized, is on the side that is clearest of fog. 

Karafuto likewise is foggy. In summer, its east 
and south coasts sometimes have continuous fog for 
days. In winter, fog is much less frequent and occurs 
mostly in patches. Fog accompanies southwesterly 
and easterly winds, and is less frequent on the west 
side of the island than on the east. 

Temperature and Icing 

Located off the east (leeward) coast of Asia, the 
Japanese Islands partake of continental conditions 
and experience much greater extremes of tempera¬ 
ture than do the American Pacific coasts. Cli¬ 
matically, Japan can properly be compared with the 
east coast of North America from Florida to New¬ 
foundland. Four broad temperature zones may be 
recognized: (1) Taiwan, the Ryukyu, and southern 
Kyushu, where frosts almost never occur; (2) south¬ 
ern and central Japan Proper, where the average 
temperature of the coldest month is above freezing; 
(3) northern Honshu and Hokkaido, cold temperate 
lands with cold winters and hot summers; and (4) 
Karafuto and the Kurils, a subarctic zone with 
cool summers and very cold winters. 

The southern islands, like Florida, have a sub¬ 
tropical climate. Highest summer temperatures ap¬ 
proach ioo°F. The mean temperatures for the coldest 
month range from 40 °F. to 50 °F.; and the mean 
of the warmest month is about 80 °F. In summer, 
with high surface temperatures, the base of the icing 
zone is above normal flying levels. During winter 
outbursts of cP air from the continent, however, 
the icing level may drop to below 3000 feet. Owing 
to the great water content of the atmosphere over 
these subtropical seas, it is highly probable that icing, 
if encountered, would be rather severe. 

Southern and central Japan have warm temperate 
conditions like those of our southeast coast. Average 
temperatures of Tokyo (Jan., 38°F.; July, 76°F.) 
compare favorably with those of Richmond, Vir¬ 
ginia, or Nashville, Tennessee. Moderate to severe 
icing may be expected occasionally in this section. 
Icing at normal flying levels will be severe in the 
spring and fall during the periods of greatest cyclonic 
activity. It will be most severe during the passage of 
warm fronts at critical temperatures (15 °F. to 32 °F.) 
and immediately after the passage of cold fronts. 
In winter, icing is a serious handicap on west coasts, 
while on east coasts the low-average winter cloudi¬ 


ness materially reduces icing hazard. Icing is very 
severe in mT air which occasionally invades central 
Japan in winter; but in summer, when this air mass 
is most common over the area, the high temperatures 
ordinarily cause the icing zone to be above normal 
flying levels. 

Northern Honshu and Hokkaido are similar to 
New England. In summer the weather is delightful 
but with occasional high temperatures (95 °F. to 
ioo°F. extremes) in July and August. Winter has 
deep snows, stormy weather and cold temperatures. 
Though the winter temperatures sometimes rise to 
60°F. or 70°F., extreme winter temperatures often fall 
below zero, and inland stations have recorded forty 
below. Temperatures below freezing may occur 
from October to May. With freezing temperatures 
common at the surface, icing is a frequent hazard 
at all except highest levels. It will be most severe 
on the west coasts with their high percentage of 
cloudiness and precipitation. 

Karafuto and the Kuril Islands may be compared 
with Newfoundland. Not only do continentality 
and icebound shores result in cold winters, but cold 
ocean currents keep summers cool and foggy. In 
summer, the temperature is nearly uniform at all 
points, the August mean being about 62 °F. In 
winter, the February average ranges from i9°F. in 
the Kurils to 2°F., in northern Karafuto. Extreme 
temperatures of -50°F. have been recorded; and, as 
winter winds are usually strong, the cold is felt 
severely. Subfreezing temperatures have been re¬ 
corded in all months except August. 

Icing conditions can be encountered in this north¬ 
ern section at all seasons of the year. In winter, with 
surface temperatures around io°F., it is safe to 
assume that at elevations above 3000 feet the air 
will be too cold for ice (other than rime) to form; 
but surface and low-level icing may be quite severe. 
During the spring and autumn, icing is at its worst, 
because frontal activity increases and surface tem¬ 
peratures are near the freezing point. Ahead of the 
fronts, warm moist air increases the amount of icing 
and the height of the icing level. Because of this, 
it is often necessary to fly at even higher altitudes 
in spring and autumn than in winter to avoid heavy 
glaze ice. In summer, the icing level is generally 
above 5000 feet. Most of the clouds over the area 
are stratus and strato-cumulus. Occasional cumulus 
clouds will develop, but not to any great height. 
The average top of clouds over the water is less 
than 5000 feet. In summer, then, it is possible to get 
on top with little icing. 


181 



WEATHER MAPS OF EASTERN ASIA AND JAPAN 


Figures 176-187 show sample weather maps for 
eastern Asia and Japan during the four seasons of the 
year. The reports of observations are usually not as 
complete as you are accustomed to receiving, so 
more and more you are dependent upon your knowl¬ 
edge of general weather to supply the missing data. 
For example, within the United States, when the 
weather map does not carry a ceiling figure, you 
interpret it to mean an unlimited ceiling; but for 
weather maps of foreign lands (and waters) you 
must supply the missing data. 

Winter 

The winter series of weather maps, January 18, 
19, 20, 1937, (Figures 176, 177, 178) illustrates a 
typical situation of low pressures developing and 
moving along the polar front. At this time of year 
this front lies off the coast of Asia and east of Japan 
at the meeting of the cold cP air from the continent 
and warm mT air from the ocean. On January 18th 
(Figure 176) you will notice that a shallow low 
(1014 mb.) lies over the East China Sea between 
Taiwan and Japan. The warm front of this depression 
has brought widespread precipitation to the north and 
northwest of the low’s center and, while no ceiling 
or visibility reports were transmitted, it is safe to 
say that they were low and that icing would also be 
a flight hazard. On January 19th (Figure 177) the 
low deepened to 1002 mb. and moved northeastward 
in a normal path, the warm front continuing to 
bring rain and snow over a large area including 
Japan. A stationary front lies over Taiwan bringing 
some bad weather to that area. Frontal and pre¬ 
frontal showers are evidenced along the cold front. 
On January 20th (Figure 178) the low-pressure area 
of January 18th has gone the way of all well-man¬ 
nered Pacific cyclones—up to the Aleutians—and we 
see another wave developing along the polar front in 
the vicinity of the Ryukyu Islands. Notice another 
interesting feature shown on these maps—fogs are fre¬ 
quent on the northern and eastern coasts of Indo- 
China (probably caused by the subsidence of cold 
air aloft hitting the warmer moist air of maritime 
origin). 

Spring 

The weather maps for April 13, 14, 15, 1938, 
(Figures 179, 180, 181) are examples of spring situa¬ 


tions. The high pressure over Asia has weakened, 
the polar front lies more to the northwest than in 
winter, and, all in all, things begin to happen faster. 
The weather map for April 13th (Figure 179) shows 
a low pressure with a long cold front moving off to 
the northeast and another frontal system lying over 
Asia in the trough between two highs. Nice weather 
exists over Japan except for isolated thunderstorms 
and some high winds. On April 14th (Figure 180) a 
deep low (990 mb.) has developed along the polar 
front over eastern Manchuria, while another warm 
front extends eastward from the cold front near the 
coast of China. By April 15th (Figure 181) the lows 
developing along the front in Asia (Figure 179) have 
become very intense (984 mb. and 982 mb.). They 
are now centered off the coast of Asia and over Japan, 
giving the entire cyclonic area considerable stormi¬ 
ness with rain and high winds. Several stations report 
dust storms. 

Summer 

As you have already read earlier in this text, dur¬ 
ing the summer the polar front has moved far to the 
north in this area and we now have a coastal front. 
Also you know that this coastal front may not move 
seaward for days but may be just a series of stagnat¬ 
ing lows for extended periods. The weather maps 
for July 5, 6, 7, 1937, (Figures 182, 183, 184) illus¬ 
trate the summer situation. Notice how the coastal 
front just seems to shift around on these three maps 
but doesn’t seem to go anywhere, the waves develop¬ 
ing and occluding out within 24 hours. Some frontal 
rains occur along the coasts and over the islands. 

Autumn 

The autumn series of weather maps for this area, 
October 7, 8, 9, 1937, (Figures 185, 186, 187) shows 
that summer conditions still prevail but the coastal 
front has now moved farther southward and east¬ 
ward, and the polar front has also moved far enough 
south to make an appearance on this map. The lows 
developing along the coastal front to the south of 
Japan maintain the summer characteristics of remain¬ 
ing more or less stationary and stagnating with local 
showers, but those farther north are traveling extra- 
tropical cyclones and are on the move in an easterly 
direction. 


182 



Fig. 176. Weather map, January 18, 1937 

183 


















184 


Fig. 177. Weather map, January 19, 1937 






















185 



























1002 


186 


Fig. 179. Weather map, April 13, 1938 




























Fig. 180. Weather map, April 14, 1938 


187 





























Fig. 181. Weather map, April 15, 1938 


188 



































1008 


_ 


Fig. 182. Weather map, July 5, 1937 


189 






















190 























120 ° 


140 ° 


1014 


ion 




A<** 

VzrP 4 ' 




S* 


1017 


USN 

July 7'37 

2200 GOT 

- 


Fig. 184. Weather map, July 7, 1937 


191 

























192 


Fig. 185. Weather map, October 7, 1937 
























USN 

Oct.8/37 

2200 GCT 

timi 


120 ° 


130 ° 


n n 


isi 


71S 


6 ® \ 

12 # 75 ^ 75 10 

7515 \ a-1# 


m- 12.1 


12 /.# 


74 ^ 


5 o^ tH 

c h' hA 

se A 




/f 0 

t-' 3 ° 


Fig. 186. Weather map, October 8, 1937 


193 















no 0 


120 ° 


130° 


Fig. 187. Weather map, October 9, 1937 

194 



















KNOWLEDGE IS POWER 

Your knowledge of the weather of naval operation zones will be power to you. 

You have learned here, in basic outline, the flying weather of many ocean and 
coastal areas—your potential operation zones. You have learned that the same natural 
laws which control the “home-town” weather, control all weather, everywhere—that 
wherever like climatic controls come together they create like climate, so that the world 
pattern of climatic * controls produces a world pattern of flying weather. You have 
learned that the Aleutian and Icelandic areas have summer fogs and winter gales in 
common; that the Kuril and Newfoundland areas have a combination of ocean cur¬ 
rents and summer air mass movements which make them veritable summer fog facto¬ 
ries; that the polar front of the North Atlantic and the polar front of the North Pacific 
are twin weather mills that grind furiously—turning out lots of weather (good and 
bad) and fast; and that the flying weather of the west coast of Europe closely paral¬ 
lels that of the west coast of North America. You have learned that the subtropical 
hearts of the great oceans have flying weather at near its best; that tropical cyclones 
generally occur on the west sides of oceans, moving in characteristic curves; and that 

195 





the intertropical front extends around the world near the “thermal” equator. On the 
other hand, you know that the monsoon area of the Southwest Pacific and Indian 
Ocean is unique to that area—that it has no counterpart because only in the great land 
mass of Asia do the forces that make monsoons attain such magnitude. You have 
learned these and many other aspects of the climatic pattern of earth, and the seasonal 
changes in the flying weather of the differing areas. 

You have acquired a basic outline of facts upon which you will build fuller knowl¬ 
edge from further study and talks with aerologists and flyers, and from personal 
experiences and observations. 

When on assignment in naval operation zones, you will use this knowledge to 
interpret correctly the available weather information and to interpret correctly your 
own observations while on flying missions. In combat with the enemy, weather knowl¬ 
edge is power. Remember that the reliable information you bring back to the 
aerologist will give him more power to pass on to you and to your fellow pilots. 

Do not assume that, because you have some knowledge of world weather, you have 
a monopoly on the advantages of weather knowledge. The Tokyo-Berlin war lords 
laid their war plans a long time ago and have used weather as an offensive weapon in 
practically every aggressive move. The aggressor can time his moves to fit the weather, 
while the defender must accept and use the weather as it is. We are now on the offen¬ 
sive, so we can pick our weather. But never underestimate your adversary in this respect 
—he has been trained to play the weather. He, like you, recognizes its importance. 

The better pilots invariably are students of flying weather. Know your flying 
weather and be “better” than the adversary. 


GLOSSARY 


Adiabatic—A. physical process which involves no change in 
heat content. Adiabatic changes in temperature are those that 
occur as a consequence of compression or expansion accom¬ 
panying a change in pressure. 

Adiabatic lapse rate— 5.5 °F. per 1000 feet, the rate of cooling 
of dry air when it ascends adiabatically. Conversely, if air 
descends it warms at the same rate. 

Advection— The process of transfer by horizontal motion, par¬ 
ticularly applied to the transfer of heat by horizontal motion of 
the air. 

Advection fog—Fog resulting from the transfer of warm, 
humid air over a cold surface, especially a cold ocean surface, or 
(comparatively rarely) from the transport of air that is relatively 
very cold over an ocean surface that is relatively very warm. 

Air mass— An extensive body of air within which the condi¬ 
tions of temperature and moisture in a horizontal plane are 
essentially uniform. 

Aleutian love—An area in the vicinity of the Aleutian Islands 
in which the average atmospheric pressure is low. 

Alto-cumulus— An intermediate cloud type occuring in a layer 
or patches composed of a great number of small cloudlets 
arranged in groups, in lines or in waves. It is definitely a layer 
cloud and not a heap cloud. Alto-cumulus forms between 6500 
and 20,000 feet. 

Alto-stratus— An intermediate cloud type occuring as a thick 
veil of cloud usually grey, sometimes steely or bluish, in color, 
generally covering the whole sky. The approximate height is 
about 17,000 feet and cloud may be from 1000 to 2700 feet thick. 

Anticyclone—A type of atmospheric circulation which is char¬ 
acterized by relatively high pressure at the center. The winds 
blow out of, and around the center; clockwise in the Northern 
Hemisphere, and counterclockwise in the Southern Hemisphere. 

Antitrades— The winds which lie above the trade winds and 
blow from the opposite direction (southwest in the Northern 
Hemisphere, and northwest in the Southern Hemisphere). 

Anvil cloud— Cloud having a projecting point or wedge like 
an anvil. The form is usually assumed by the tops of fully 
developed cumulo-nimbus clouds. 

Arid—A climate in which the rainfall is insufficient to support 
forest vegetation. 

Autumn— The season of the year between summer and winter. 
Usually considered to consist of the months, September, October 
and November, in the Northern Hemisphere; March, April and 
May in the Southern Hemisphere; also called Fall. 

Backing—A counterclockwise shift in the wind direction. 
Opposite of veering. 

Baguio—A local Philippine name for a tropical cyclone. 

Beaufort scale— The scale of wind force devised by Admiral 
Sir Francis Beaufort in 1805 and is the universally recognized 
scale (see p. 206). 

Bize—A cold, dry wind which blows in the winter in the 
mountainous regions of southern France from the N., NE. or 
NW. The cold NW. wind which occurs in Languedoc, near the 
Mediterranean coast, and differs from the Mistral in that it is 
accompanied by heavy clouds. 

Blizzard—A violent, intensely cold wind, laden with snow. 

Bora—A strong, cold, north wind of the gravity type, blowing 
down across Greece from the uplands of the Balkan region. 

Brickfielder—A hot, cyclonic type wind of Australia. 

Buys Ballot's Law— In the Northern Hemisphere, if you face 
the wind the atmospheric pressure decreases toward your right 


and increases toward your left. When you face the wind in the 
Southern Hemisphere, the reverse is true. 

California Current—A cold ocean current off the California 
coast which is a contributory factor in producing summer fog. 

Chinook— Same as foehn wind. 

Cirro-cumulus—A high cloud type consisting of a layer or 
patch of small white flakes or of very small globular masses, 
without shadows, which are arranged in groups or lines or more 
often in ripples resembling those on the seashore. Cirro-cumulus 
is thin and high—about 20,000 to 25,000 feet. 

Cirro-stratus—A high cloud type consisting of a thin whitish 
veil with a base at about 20,000 to 30,000 feet. The cirro-stratus 
is usually moving away from the bad weather region. 

Cirrus—A high cloud type consisting of fibrous wisps of deli¬ 
cate white cloud formed of ice crystals and showing up against 
the blue background of the sky; sometimes known as “Mares’ 
Tails”. These clouds are very high, between 20,000 and 30,000 
feet, and are frequently one of the first signs of an approaching 
warm front. 

Climate— The average weather conditions of any locality. 

Col—A neck of relatively low pressure between two anti¬ 
cyclones; also called a “saddle”. 

Cold front— The discontinuity at the forward edge of an 
advancing cold air mass which is displacing warmer air in its 
path. 

Condensation—'The process whereby water vapor is re-formed 
into liquid water; the reverse of the process of evaporation. 

Conditional instability— Air which, though stable at the sur¬ 
face, becomes unstable if lifted mechanically to the condensation 
level. 

Conduction—The transfer of heat through and by means of 
matter without any necessary movement of matter. 

Convection— The upward or downward movement, mechani¬ 
cally or thermally produced, of a limited portion of the atmos¬ 
phere. Convection is essential to the formation of many clouds, 
especially of the cumulus type. 

Convergence— The condition that exists when the distribution 
of winds within a given area is such that there is a net horizontal 
inflow of air into the area. The removal of the resulting excess 
is accomplished by an upward movement of air; consequently 
areas of convergent winds are regions favorable to the occur¬ 
rence of precipitation. 

Coriolis force— The deviating force on a particle in motion 
due to the rotation of the earth; directed to the right in the 
Northern Hemisphere, to the left in the Southern Hemisphere, 
and zero at the equator. 

Cumulo-nimbus— Heavy masses of cloud with great vertical 
development, whose cumuliform summits rise in mountainous 
towers, the upper part having fibrous texture and often spreading 
out in the shape of an anvil. They generally produce showers 
of rain or snow and sometimes hail, and often thunderstorms. 

Cumulus—A dense, dome-shaped, puffy-looking cloud of verti¬ 
cal development. The base height varies greatly and the tops 
may extend to the cirrus level. 

Cyclone— An area of low barometric pressure with its attendant 
system of winds, counterclockwise and into the center in the 
Northern Hemisphere, clockwise in the Southern Hemisphere. 
The cyclones occurring within the tropics (tropical cyclones) 
are smaller, on an average, than those of higher latitudes, and 
in many cases are the most violent of all storms, except tor¬ 
nadoes. Those occurring in higher latitudes (extratropical 


197 



cyclones) whether originating there or in the tropics, usually 
bring about marked changes of weather and temperature during 
their passage; their winds may be high or otherwise. Tropical 
cyclones when violent are also called hurricanes, typhoons, or 
baguios. Extratropical cyclones are commonly known as lows 
or barometric depressions. 

Cyclone family— A series of cyclones developing by the “wave” 
process along a polar front. 

Density— A measure of the concentration of matter. Weight 
per unit volume. 

Depression.—A cyclonic area, or low. See Cyclone. 

Dew point— The temperature at which, under ordinary condi¬ 
tions, condensation begins in a cooling mass of air. It varies with 
the specific humidity. The dew point is a conservative air mass 
property. 

Divergence— The condition that exists when the distribution 
of winds within a given area is such that there is a net horizontal 
flow of air outward from the region. The resulting deficit is 
compensated by a downward movement of air from aloft; con¬ 
sequently areas of divergent winds are regions unfavorable to 
the occurrence of precipitation. 

Doctor— A sea breeze in the tropics. 

Doldrums— The equatorial belt of calms or light variable winds, 
lying between the two trade-wind belts. 

Drizzle— Precipitation consisting of numerous tiny droplets. 
Drizzle originates from stratus clouds. 

Diurnal— Daily. 

Dynamic cooling— The fall in temperature produced by expan¬ 
sion due to diminished pressure. See Adiabatic lapse rate. 

Dust— Solid particles of varying character and size carried 
in suspension by the atmosphere, often for long distances. 

Eddy— Local irregularity in a current of flow. All winds near 
the earth’s surface contain eddies which at any given place 
produce “gusts” and “lulls”. 

Equatorial air— Air originating in the doldrum belt and the 
equatorward portions of the trades. Has high temperatures, high 
humidity, and is unstable. 

Equinox— The points in the earth’s orbit at which the sun 
crosses the equator. The vernal equinox occurs on March 21st, 
and the autumnal equinox on September 23rd, in the Northern 
Hemisphere. Day and night are equal on these dates, hence the 
name. 

Evaporation— The change from the liquid to the vapor phase 
of a substance. 

Expansion— The increase in size of a sample of material, 
which may be due to heat or to the release of mechanical strain, 
or the absorption of moisture or some other physical or chemical 
change. 

Eye of storm— The central region of approximate or actual 
calm conditions, and often clear skies, in the center of a tropical 
hurricane. The atmospheric pressure of the storm will attain 
the minimum value here. 

False cirrus— Cirrus-like clouds at the summit of a thunder¬ 
cloud; more appropriately called “thunderstorm cirrus”. 

Floe—A large mass of drifting ice from the polar ocean. 

Foehn (Fohn)—A dry wind with strong downward compo¬ 
nent, warm for the season, characteristic of many mountainous 
regions. The air is cooled dynamically in ascending the moun¬ 
tains, but this leads to condensation, which checks the fall in 
temperature through the liberation of latent heat. The wind 
deposits its moisture as rain or snow. In descending the opposite 
slope it is strongly heated dynamically and arrives in the valley 
beyond as a warm and very dry wind. Also called chinook. 

Fog— Water vapor which has condensed in the form of 


water droplets in the lower part of the atmosphere and inter¬ 
feres with its transparency. It differs from cloud only in 
being near or at the surface. It is easily distinguished from haze 
by its essential wetness. 

Front— An atmospheric discontinuity surface separating two 
dissimilar air masses. 

Fr onto gene sis— The process whereby fronts are generated by 
the meeting of air masses of different densities, or by the crea¬ 
tion of a steep temperature gradient along the horizontal, within 
an air mass, so that a temperature (and therefore density) dis¬ 
continuity occurs. 

Gale—A wind of relatively high velocity. See Beaufort scale. 

Gradient— Change of value of a meteorological element per 
unit of distance. The gradients commonly discussed in aerology 
are the horizontal gradient of pressure, the vertical gradient of 
temperature, and the vertical gradient of electrical potential. 
Aerologists now prefer the term “lapse rate” to vertical tem¬ 
perature gradient. 

Gulf Strea??i—A warm ocean current originating in the Gulf 
of Mexico and flowing along the east coast of North America. 

Gust—A sudden brief increase in the force of the wind. Most 
winds near the earth’s surface display alternate gusts and lulls. 

Haze—A lack of transparency in the atmosphere caused by 
the presence of dust or of salt particles left by evaporated ocean 
spray. 

Harmattan—A very dry wind which is prevalent in western 
Africa during the dry season (November to March). Often 
carries dust. 

High— An area of high atmospheric pressure. An anticyclone. 

Horse latitudes— The belt of high pressure calms, light winds 
and fine clear weather between the trade-wind belts and the 
prevailing westerly winds of higher latitudes. Located within the 
subtropical high-pressure belt. 

Humidity— The moisture content of the atmosphere. When 
used without a qualifying adjective this term usually refers 
to relative humidity. 

Hurricane— An extremely violent cyclonic storm of the 
Tropics. 

Icelandic low— An area in the vicinity of Iceland in which the 
average atmospheric pressure is low. 

Instability—A state in which the vertical distribution of tem¬ 
perature is such that an air particle will tend to move away 
with increasing speed from its original level. 

Instability showers— Showers due to vertical instability. Such 
instability is caused by warming of the lower layers of air or by 
a cooling of the upper layers or by a combination of the two 
causes. 

Intertropical front— The front that forms in the equatorial 
belt of convergence. 

Inversion (temperature)—A reversal in the normal tempera¬ 
ture lapse rate, in which the temperature rises with increased 
elevation, instead of falling. 

Isobar—A line on a chart drawn through places or points 
having the same barometric pressure. 

Isotherm—A line on a chart drawn through places or points 
having the same temperature. 

Japan Current—A warm ocean current flowing northward 
along the east coast of Asia. 

Kamchatka Current—A cold ocean current flowing south along 
the northeast coast of Asia. 

Katabatic—A down-slope wind of the gravity type. 

Kona—A storm developing as a regeneration of an occluded 
front in the vicinity of the Hawaiian Islands. 

Kuroshio (Kuro-Shiwo)—Same as Japan Current. 


198 


Land breeze— A (usually) light breeze blowing offshore at 
a time when the land is cooler than the ocean. 

Lapse rate— The rate of decrease of temperature in the atmos¬ 
phere with height. 

Line squall—A. more or less continuous line of squalls and 
thunderstorms marking the position of an advancing cold front. 

Looming— The coming into sight of objects normally below 
the horizon because of refraction of light in passing from denser 
air at the surface to less dense air aloft. 

Manrmato-cunmlus—A form of cloud showing pendulous sack¬ 
like protuberances. 

Mares' tails—A feathery, spreading cirrus cloud. 

Mirage— An optical effect caused by refraction of light at 
the surface between two strata of air of different temperatures. 
These effects include the illusion of a sheet of water in deserts 
as well as cases at sea in which definite distant objects can be 
seen in duplicate or one image inverted. 

Mist— In England, a very light fog. In North America, often 
used synonymously with drizzle or fine rain. 

Mistral—A cold gale along the French-Italian Riviera. Similar 
to the norther of the United States. 

Monsoon—A wind that reverses its direction with the season, 
blowing more or less steadily from the interior of a continent 
toward the sea in winter, and in the opposite direction in 
summer. 

Mountain breeze—A breeze blowing at night from a mountain 
to an adjoining valley. Partially due to temperature differences 
between the cool mountain and the warmer valley, and partially 
due to gravity flow of the cold air down the mountain slopes. 

Nimbus— An older term for clouds from which precipitation is 
falling, together with the low clouds beneath. Now superseded 
by the terms nimbo-stratus, or cumulo-nimbus. 

Norther—A term used in the Gulf Coast region of the United 
States and the Philippines for a cold gale from the north. Formed 
by a vigorous outbreak of continental polar air behind a cold 
front during winter. 

Occlusion or occluded front— The front resulting when a cold 
front overtakes a warm front either undercutting or overriding 
the cold air below the warm front. 

Ocean current—The movement of ocean waters in streams in 
a general direction over long distances. Not to be confused with 
tides. 

Orographic— Pertaining to mountains, as orographic rain 
caused by winds forced over a mountain barrier. 

Polar air— Air originating in polar regions. 

Polar continental air— Air mass originating over polar margins 
of continents in Northern Hemisphere. Cold, dry and stable in 
winter; cool, dry and unstable in summer. 

Polar maritime air—Polar air modified by reason of its passage 
over a relatively warm ocean surface or air originating over 
polar margins of oceans. It is moderately cold, moderately moist, 
and unstable in winter; cool, moist and relatively stable in 
summer. 

Polar high— Areas of high pressure over Greenland, the Arctic 
Ocean and Antarctic from which the surface air flows out anti- 
cyclonically. This air flow is deflected to the west, becoming 
northeast winds in the Northern Hemisphere and southeast 
winds in the Southern Hemisphere known as polar easterlies. 

Polar front— The surface of discontinuity separating an air 
mass of polar origin from one of tropical origin. 

Radiation— Emission of energy in the form of waves, either 
light or heat or both. 

Radiation fog— Fog characteristically resulting from the radia- 


tional cooling of air near the surface of the ground on calm, 
clear nights. 

Recurvature of storm— The recurvature of the track of a 
tropical cyclone, which is a typical feature of the great majority 
of these phenomena. 

Relative humidity—The ratio, expressed in per cent, of the 
actual amount of water vapor in a given volume of air to the 
amount which would be present were the air saturated. 

Revolving storm—A name sometimes applied to tropical 
cyclones. 

Ridge—A relatively narrow extension of an anticyclone or 
high-pressure area as shown on a weather chart. 

Roaring Forties—A nautical expression used to denote the 
prevailing westerly winds of temperate latitudes (below 4o°S.) 
in the oceans of the Southern Hemisphere. 

Sirocco—A hot southerly wind blowing from the desert 
of North Africa reaching the Mediterranean as a very hot, dry 
wind, often heavily dust-laden. On crossing the sea it becomes 
excessively humid, may reach gale force in the Adriatic and 
cause heavy rains. Fog frequently forms in the northern Medi¬ 
terranean. 

Scud—A popular name for the low, drifting clouds which 
often appear beneath a cloud from which precipitation is actively 
falling. The official name for these clouds is fracto-stratus, or 
fracto-cumulus, depending upon their exact form. 

Sea breeze—A thermally-produced wind blowing from the 
cool ocean surface onto the adjoining warm land, in the 
afternoon during summer. 

Secondary cold front—A cold front which forms behind 
the main cold front during periods of strong temperature and 
pressure gradients. Occurs only during the cold season. 

Secondary depression—A small area of low pressure on the 
border of a large or “primary” one. The secondary may develop 
into a vigorous cyclone while the primary center disappears. 

Southerly buster— In south or southeast Australia, a sudden 
change of wind from a northeasterly direction to southerly or 
southeasterly, which is accompanied by a sudden drop in tem¬ 
perature. Similar to our line squall and most prevalent from 
October to March. 

Specific humidity—Mass (weight) of water vapor contained 
in a unit mass of air. 

Spring— The season between winter and summer—March, April 
and May in the Northern Hemisphere; September, October and 
November in the Southern Hemisphere. 

Squall—A sudden, violent rain or snow storm accompanied 
by strong, gusty winds. 

Stability—A state in which the vertical distribution is such that 
an air particle will resist displacement from its level. 

St. Elmo's fire—A luminous brush discharge of electricity 
from elevated objects. It is often seen on the wing tips and 
propellors of aircraft flying in or near thunderclouds. 

Storm—A marked disturbance in the normal state of the 
atmosphere. 

Strato-cumulus— Large, lumpy masses or rolls of dull grey 
cloud, frequently covering the whole sky. 

Stratus—A low uniform layer of cloud, resembling fog, but 
not resting on the ground. 

Stratosphere— The upper portion of the atmosphere, above 
the troposphere. Its lower limit varies from about 26,000 to 
70,000 feet; its upper limit lies at about 60 miles. The base of 
the stratosphere marks an upper limit to the general corrective 
activity of the troposphere. Air motion within the stratosphere 
is largely horizontal. 


199 








Subsidence— The sinking and spreading out of a body of air, 
usually within an anticyclone. 

Sumatra— A squall which occurs in the Malacca Straits usually 
blowing from the southwest. It is accompanied by a character¬ 
istic cloud formation—a heavy bank or arch of cumulo-nimbus 
which rises to a great height. Sumatra squalls are more frequent 
at night, and between April and October. They are generally 
accompanied by thunder and lightning and torrential rain. 

Summer— The warmer season of the year. Generally June, July 
and August in the Northern Hemisphere; and December, 
January and February in the Southern Hemisphere. 

Swell— A wave motion in the ocean persisting after the 
originating cause of the wave motion has ceased or passed away. 
It often so continues for a considerable time with unchanged 
direction, as long as the waves travel in deep water. 

Thunderstorm— Any storm which is accompanied by thunder. 

Tide— The periodic rise and fall, or ebb and flow, of the 
oceans and bodies of water connected with them, due to the 
attraction of the moon and sun. 

Tornado—( i) A very intense, sharply-defined, funnel-shaped 
storm of the United States prairies. The most violent and 
sharply defined of all storms. (2) In West Africa, a violent 
thundersquall. 

Trade Winds— The winds which blow from the subtropical 
high-pressure belts toward the equatorial region of the low 
pressure, from the NE. in the Northern Hemisphere and SE. 
in the Southern Hemisphere. 

Tropical air— See Equatorial air. 

Tropical cyclone—Set. Hurricane. 

Tropopause— The boundary between the troposphere and the 
stratosphere. 

Troposphere—The. lower part of the atmosphere lying be¬ 
tween the surface of the earth and the tropopause. Marked by 
a steady average decrease of temperature with altitude, and 
considerable turbulence. 

Turbulence— Irregular motion of the atmosphere produced 
when air flows over a comparatively uneven surface, such as 
the surface of the earth, or when two currents of air flow past 
or over each other in different directions or at different speeds. 

Tundra— Treeless plains bordering the arctic coasts of Eu¬ 
rope, Asia and North America. 

Twister—A popular name for a tornado. 

Typhoon— The name applied in the Far East to a tropical 
cyclone. See Hurricane. 

Upper air— That part of the atmosphere which is not in close 
proximity to the earth’s surface. 


Valley breeze—A breeze which during the day blows up 
valleys and mountain slopes. 

Veering— The clockwise change in the wind direction. 

Warm fro?it— The advancing edge of a warm mass of air. 

Waterspout—A whirlwind over water, formed on hot days, in 
which water droplets and cloud particles are carried some dis¬ 
tance (from a few feet to several hundred feet) into the air. 
In rare cases a tornado may appear over the water and give 
rise to an exceptionally severe form of waterspout. 

Waves {ocean)—A disturbance of the surface, caused by 
wind, resulting in regular periodic ridges and troughs. 

Waves ( cyclonic ) or u wave disturbance"—A localized de¬ 
formation of a front, which travels along the front as a wave¬ 
shaped formation, and which generally develops into a well- 
marked cyclone. 

Weather— The state of the atmosphere at a given moment with 
respect to temperature, moisture, cloudiness, precipitation, or 
other meteorological phenomena. 

Wedge— (1) A wedge-shaped area of high barometric pres¬ 
sure as shown on a weather chart. Synonymous with “ridge”. 
(2) Applied to an air mass whose advancing forward position, 
from a three-dimensional standpoint, is shaped like a wedge. 

Westerlies— The prevailing winds of the middle latitudes (40° 
to 65°N. Latitude; 35° to 65°S. Latitude). In this belt the winds 
are variable in both direction and strength, largely as a result 
of the procession of fronts and storms (cyclones and anti¬ 
cyclones) that travel from west to east in these latitudes. 

Whirlwinds—A rapidly whirling vortex of air, with its axis 
vertical or nearly so, usually seen on hot, still days. The diam¬ 
eter and height may vary from a few feet to several hundred 
feet. Characterized by the inflow of air at the base and a corre¬ 
sponding outflow aloft. 

Williwaw—A cold, gravity (Bora-type) wind of the Aleutians, 
Alaska, the Strait of Magellan and Tierra del Fuego. 

Willy-willy—A tropical cyclone off the northwest coast of 
Australia. See Hurricane. 

Wind aloft— The winds above the earth’s surface. All winds 
over a station except surface winds. 

Wind rose—A diagram which shows for a given locality or 
area the frequency and strength of the wind from various 
directions. 

Winter— The cold season of the year. Generally considered 
December, January, and February in the Northern Hemisphere 
and June, July and August in the Southern Hemisphere. 


200 


BEAUFORT SCALE (With Adaptations) 



















201 









































































HOW TO PRONOUNCE THOSE PLACE NAMES 


Key to Pronunciation 

Capital letters show the accented syllable. 
Syllables are separated by dashes. 

Pronounce: 

a before a consonant in the same syllable.as 

a in hat 


Aleutians 

uh-LOO-shunz 

Antarctica 

ant-ARK-tik-uh 

Antilles 

an-TILL-eez 

Arabia 

uh-RAY-bee-uh 

Arctic Ocean 

ARK-tik 

Argentina 

ahr-j en-TEE-nuh 

Arkhangelsk 

ahr-HAHN-ghelsk 

Ascension Island 

uh-SEN-shun 

Azores 

uh-SOHR-esh (English: 
uh-ZAWRZ) 

Bahama 

buh-HAH-muh or buh- 
HAY-muh 

Bali 

BAH-lee 

Bengal 

beng-GAWL 

Bering Sea 

BAIR-ing 

Bothnia Gulf 

BOTH-nee-uh 

Calcutta 

kal-KUT-uh 

Canary Islands 

kuh-NAIR-ee 

Canton 

KAN-TONN 

Cape Verde 

VURD 

Caribbean Sea 

kuh-RIB-ee-un or karri- 
BEE-un 

Cebu 

say-BOO 

Ceylon 

see-LON 

Chile 

CHEE-lay or CHILL-ee 

Chishima (Kuril) 

CHEE-shee-mah (Kuril) 

Chosen (Korea) 

CHOH-sen (Korea) 

Cordova 

KAWR-doh-vah 

Edinburg 

ED-in-bur-uh 

Ellice Island 

EL-iss 

Eurasia 

yoo-RAY-zhuh or yoo- 
RAY-shuh 

Fiji 

FEE-jee 

Formosa (Taiwan) 

202 

fawr-MOH-suh (TIGH- 
wahn) 


APR121B50 


e before a consonant in the same syllable.as 

e in let 

i before a consonant in the same syllable.as 

i in sit 

o before a consonant in the same syllable.as 

o in hot 

u before a consonant in the same syllable.as 

u in cut 


Galapagos 

gah-LAH-pah-gohs 

Ganges 

GAN-jeez 

Glasgow 

GLAS-goh 

Guam 

GWAHM 

Guatemala 

gwah-teh-MAH-lah 

Guinea Coast 

GHIN-ee 

Habana 

ah-BAH-nah (huh 
VAN-uh 

Haiti 

HAY-tee 

Hammerfest 

HAM-ur-fest 

Hawaii 

huh-WIGH-ee 

Himalayas 

hih-MAHL-uh-yuhz 

Hokkaido 

hok-KIGH-doh 

Hongkong 

HONGKONG 

Honolulu 

HON-uh-LOO-loo 

Honshu 

HON-shoo 

Horn 

OR-tah 

Hwang Ho 

HWAHNG HOH 

Iberia 

eye-BEER-ee-uh 

Jamaica 

juh-MAY-kuh 

Jan Mayen 

YAHN MIGH-un 

Java 

JAH-vuh 

Juneau 

JOO-noh 

Kamchatka 

kum-CHAT-kuh 

Karafuto 

kah-RAH-foo-toh 

Kauai 

KOW-igh 

Kiska 

KISS-kuh 

Kodiak 

KOH-dee-ak 

Koepang 

KOO-pahng 

Korea (Chosen) 

kuh-REE-uh (Chosen) 

Kuril (Chishima) 

KOO-ril (Chishima) 

Labrador 

LAB-ruh-dawr or lab- 
ruh-DAWR 

Liman Current 

LEE-mun 

Luzon 

loo-ZON 










Malacca 

muh-LAK-uh 

Malta 

MAWL-tuh 

Manchuria 

man-CHOO-ree-uh 

Manila 

muh-NIL-uh 

Matanuska 

mat-uh-NOOS-kuh 

Murmansk 

moor-MAHNSK 

Natal 

nuh-TAHL 

Newark 

NOORK (local in N. J.) 
or NOO-urk (NEW- 
urk) 

Newfoundland 

NOO-fund-lund or noo- 
fund-LAND 

Niitaka 

nee-EE-tah-kah 

Nome 

NOHM 

Okhotsk 

uh-HAWTSK 

Oporto 

oo-POHR-too (oh- 
PAWR-toh) 

Pago Pago 

PAHNG-oh-PAHNG- 

oh 

Petropavlovsk 

pit-ruh-PAH V-lufsk 

Phoenix Islands 

FEE-niks 

Rabaul 

rah-BAH-ool 

Rangoon 

rang-GOON 

Reykjavik 

RAY-kyuh-vik 


Rhone 

ROHN 

Riviera 

riv-YAIR-uh 

Ryukyu 

RYOO-KYOO 

Sahara 

suh-HARRuh or suh- 
HAH-ruh 

Sakhalin 

sah-hah-LEEN 

Samoa 

suh-MOH-uh 

San Juan 

SAHN HWAHN 

Shikoku 

shih-KOHK-koo 

Sitka 

SIT-kuh 

Stranraer 

strahn-RAHR 

Sudan 

soo-DAHN or soo-DAN 

Sumatra 

soo-MAH-truh 

Taiwan (Formosa) 

TIGH-wahn (Formosa) 

Terakan 

tah-rah-KAHN 

Timor 

tee-MOHR 

Tokyo 

TOH-kyoh 

Tonga 

TONG-guh 

Tsusima 

TSOO-shee-mah 

Waialeale 

wigh-ah-lay-AH-lay 

Yangtze Kiang 

YAHNG-tsuh 

Yokohama 

yoh-kuh-HAH-muh 




203 






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